As Donald Trump enters his fourth year in office, there is still a blind spot at the heart of his presidency.

The blind spot is Trump’s business, which he still owns but divulges little about.

Through the Trump Organization, the president has a web of relationships with customers, lenders and investors that remains largely out of sight. That could mean President Trump is exposed to problems — such as struggling properties or debts coming due — that would put private pressure on a man with immense public power.

We know a bit more about that business after this past year of reporting but plan to dive deeper in the coming year.

Among the things we learned in 2019: Some of the Trump Organization’s key properties are struggling. The company’s brand has become toxic to an array of customers. The company has pushed out dozens of undocumented workers it was employing. And lawsuits surrounding Trump’s possible conflicts of interests are piling up.

Trump also showed a greater willingness last year to boost his business from the White House, the most stunning move being a decision to award the Group of Seven summit to his own club. Trump later backed off that decision under pressure.

This is what we will look to learn about the Trump Organization in 2020:

1. How will the company adapt now that it has let go of many undocumented immigrant workers?

As a politician, Trump talks tough about stopping illegal immigration. As a businessman, it has been a different story.

One thing we learned last year is that Trump’s business was built with the help of the labor of undocumented immigrants, even as Trump denounced them in public.

The Washington Post spoke with nearly 50 undocumented workers who had been employed by the Trump Organization, some for as long as two decades, across 11 of the company’s golf courses and resorts in the United States. They drove machines that built fairways and greens. They cleaned clubhouses and the Trump family’s personal villas. They cooked Trump’s burgers the way he liked them and carried the food to his table.

And they drove tractors and picked grapes at Trump’s winery in Charlottesville, where Trump’s company fired at least seven workers on Dec. 30, more than a year after the Trump Organization’s reliance on the undocumented first came to light.

These workers said they suffered many of the same problems millions of undocumented immigrants face in the United States: lower pay, a lack of benefits, shortchanged hours and the precarious sense they could not stand up for themselves to their employers without risking their jobs.

We still do not have a full accounting of how many undocumented workers the Trump Organization employed or how many people it fired after these revelations came to light.

Since reports on the Trump Organization’s undocumented employees appeared, the company has instituted new measures to verify applicants’ documents.

Eliminating these workers from the payroll could mean the company has to increase salaries or offer more benefits to attract replacements who are legal to work. That, in turn, could further strain the company’s finances. How the Trump Organization moves past revelations about its undocumented workforce will be key in 2020.

2. How is Trump’s business doing financially?

Last year, The Post revealed that three of Trump’s key properties — in Miami, Chicago and Washington — were lagging behind their competitors. Those three properties carry more than $300 million in debt, a large chunk of Trump’s outstanding loans, according to financial disclosures he files annually with the government.

In Miami, the Trump Organization’s filings with local tax authorities showed that profitability at the Trump Doral resort had fallen 69 percent from 2015 to 2017.

At Trump’s hotel in Chicago, the company reported an even steeper drop in profits last year: 89 percent.

The reason? Trump’s tax consultants blamed politics, saying Trump’s politicized brand seemed to be driving customers away. In Chicago, they cited a quote from a Post report in which an investor called Trump’s foray into politics an “embarrassment.”

There was other bad news: Trump’s golf course in New York City lost money. Revenue at his Mar-a-Lago Club in Florida dropped 10 percent. Apartment buildings in Manhattan stripped off Trump’s name. Trump golf courses in Scotland and Ireland all reported financial losses for the year, again.

The company’s planned expansion of its hotel operation with two new lines of lower-cost hotels fizzled. Early last year, the company canceled its plans for “Scion” and “American Idea” hotels, saying that the hotels would inevitably be dogged by concerns about conflicts of interest.

But the biggest news was in Washington, where Trump’s D.C. hotel had seemed like a bright spot, buzzing with Republican allies. Behind the scenes, the hotel was lagging behind its peers in occupancy rates and room revenue, according to Trump Organization documents obtained by The Post.

This fall, the Trump Organization made a surprising decision: It said it was exploring a sale of the D.C. hotel’s lease.

3. Will selling be a new strategy?

The company’s old plan had been to put itself in a kind of stasis during Trump’s presidency: It would stop seeking large-scale deals and just run its consumer-focused businesses. The company said this arrangement would limit the potential for any conflicts of interest: Any new customers Trump attracted would be small potatoes. A new club member, a new hotel guest — nothing big enough to raise concerns about potential corruption.

This arrangement “effectively put me out of work,” Donald Trump Jr. wrote in his book “Triggered.” Trump Jr. was theoretically running the business with his brother, Eric. But with no new deals to seek, Trump Jr. seemed to indicate that it did not need much running: “All that was left for me to do was spend my time campaigning for my father.”

But the auto­pilot plan works only if the existing Trump businesses can sustain themselves.

In Washington, the company is reportedly asking $500 million for the Trump hotel, which would be an unusually high price for a luxury D.C. hotel.

Trump Jr. has said one reason for seeking a sale is concern about conflicts of interest and the lawsuits filed over its foreign-government customers. But the sale itself could present a far larger potential for conflict — it could allow a single person or company to pay the president’s company a massive lump sum.

In Europe, the Trump Organization’s plan is to start selling homes.

The Trump Organization won approval from local councils last year to build more than 550 homes in Scotland and Ireland. Many of the homes that the company wants to build are high-end residences that could attract foreign buyers.

The home-building strategy will be a big new challenge for the company, especially in such fraught political times. At one of Trump’s Scottish golf clubs, he faces neighbors determined to be a thorn in his side, to preserve their pastoral lifestyle and to prevent crowded roads and schools. At the other Scottish course, a local council has blocked the company’s construction drive thus far.

None of the European courses have made a profit in the years since Trump purchased them. The combination of golf tourism and private events has not been enough to get over the hump. Selling vacation homes might be, but it is still a long road to get there.

4. How is Trump changing his tactics when it comes to promoting or distancing himself from his business?

In October, acting White House chief of staff Mick Mulvaney made a stunning announcement. Trump had awarded the contract to host next year’s Group of Seven summit to himself.

The summit would be at Trump National Doral Miami, Mulvaney said. That would mean hundreds of guests, and a lot of federal spending, all arriving during the hotel’s traditional slow month of June.

Mulvaney said Trump did not intend to profit.

“He has no interest in profit from being [president],” Mulvaney said.

But the move was so concerning that even Republicans objected, and Trump canceled the plan a few days later. The summit was moved to Camp David, the presidential retreat in Maryland (which Mulvaney had earlier dismissed as “a miserable place to have it”).

But beyond that episode, Trump also took smaller steps that used the power of the presidency to boost his business. He attended GOP fundraisers at four Trump properties, meaning that he was simultaneously raising money as a candidate and taking in revenue for ballroom rental and catering. Trump visited his Doonbeg resort in July — taking in $110,000 selling food to the Irish police sent to protect him — and then, according to Vice President Pence’s chief of staff, encouraged Pence to visit the same resort a few months later.

Trump also used his Twitter account to tout one of his golf courses and his for-profit Mar-a-Lago Club — where membership reportedly costs $200,000.

“I will be there in two weeks, The Southern White House!” Trump wrote on Twitter in December.

5. Who will try to influence the Trump administration by patronizing Trump’s businesses?

Last year, The Post obtained “VIP Arrivals” logs that the Trump hotel in Washington handed out to prepare its staff for arriving guests. They showed that some of the hotel’s most valued customers also wanted something from the Trump administration.

In April 2018, for instance, wireless providers T-Mobile and Sprint announced a landmark merger that needed approval from the Trump administration.

“It’s become a place I feel very comfortable,” Legere said when a Post reporter found him in the Trump hotel’s lobby.

T-Mobile denied it was seeking influence. After the Post report appeared online, Legere moved to the Four Seasons.

Trump administration officials did not comment about the executives’ hotel stays. Trump’s Justice Department later approved the merger.

The VIP Arrivals logs also showed another big-spending guest who wanted influence in Washington. Nahro al-Kasnazan, a wealthy Iraqi who wants the United States to take a hard line against Iran, stayed 26 nights in one of the hotel’s suites in 2018.

Kasnazan said he did not expect that stay — which probably cost tens of thousands of dollars — to buy him influence in Trump’s Washington. But he still used his trip to socialize with State Department officials and to raise his profile among people in Trump’s orbit.

“We saw all the Trumpers,” his spokesman said.

The Trump Organization said it had donated the profits from Kasnazan’s stay but did not say how much those were.