Donald Trump's second-youngest child Tiffany is a 22-year-old recent college grad. She spoke about her father's softer side at the Republican National Convention. (The Washington Post)

All four of Donald Trump's adult children will speak at the Republican National Convention this week. Three of them have become familiar faces on the campaign trail. One remains a relative unknown.

Tiffany Trump, the youngest of the four, recently graduated from the University of Pennsylvania. Tuesday's speech will mark the 22-year-old's first major outing on the campaign trail -- and it will be in prime time, right after New Jersey Gov. Chris Christie.

Here's what you need to know.

She's big on Instagram

Only appropriate transportation in this #blizzard #snowplow ☃❄️

A post shared by Tiffany Ariana Trump (@tiffanytrump) on

As The Post's Emily Heil reported last week:

Although Tiffany Trump isn’t a regular in front of the cable TV cameras, she’s already famous — or at least Internet famous, and that counts, right? The recent University of Pennsylvania graduate has an enviable 156,000 followers on Instagram who follow her adventures among the infinity pools and ski slopes that constitute the backdrop for her young-and-fabulous set.

A New York Times story recently dubbed Tiffany Trump’s gaggle of social-media savvy fellow children-of the the”Snap Pack”: the gang includes RFK Jr.’s daughter Kyra Kennedy, Stephanie Seymour’s son Peter Brant Jr., and Gaia Matisse, the great-great granddaughter of painter Henri Matisse.

To put her IG following in perspective, that’s a lot less than her father (with 1.9 million) but enough to qualify her, in the parlance of ‘gramming experts, as a“mid to top-tier influencer,” on par with the likes of minor reality TV stars.

A sampling:

Let the weekend commence 🎉 #fbf

A post shared by Tiffany Ariana Trump (@tiffanytrump) on

👌🏻🏁 #audibrooklyn

A post shared by Tiffany Ariana Trump (@tiffanytrump) on

👄

A post shared by Tiffany Ariana Trump (@tiffanytrump) on

She's the only child from Trump's second marriage, and grew up with her mother

Tiffany Trump's three older siblings are actually half-siblings, born to Trump's first wife, Ivana Trump. Tiffany is the daughter of Trump's second wife, Marla Maples. The mother of Trump's youngest child Barron is Melania Trump, the nominee's third wife.

Tiffany also grew up in California -- with her mother, to whom she's very close -- and not in New York with the rest of her siblings.

“I don’t know what it’s like to have a typical father figure,” she told Dujour. “He’s not the dad who’s going to take me to the beach and go swimming, but he’s such a motivational person.”

Maples has described herself as raising Tiffany Trump as a "single mother."

She released a song in 2011

It's called "Like A Bird (feat. Sprite & Logic)." It gets 3 our of 5 stars on Amazon. A sampling of the reviews:

"Normally, I don't write reviews on music, however I would not be writing this review if I thought it was bad. I have to say I enjoyed it. It’s a very catchy song."

Others thought it was too heavily auto-tuned. #millennials

She has stood up to those accusing her dad of sexism

One of her few political actions was in April when she stood up for her dad amid accusations of sexism during a family appearance at a CNN town hall.

"He wants us to do the best, and he has the utmost faith that we can accomplish whatever set our minds to just as well as men, if not better," she said.

She dabbled in the family business

Modeling, that is. Ivana Trump, Melania Trump and Ivanka Trump -- Tiffany's older half-sister -- have all been models. Tiffany's debut was earlier this year, and she has interned for Vogue magazine.

So proud of @adwarren for his debut of @justdrewclothing at #NYFW

A post shared by Tiffany Ariana Trump (@tiffanytrump) on

Her mother tweeted and Instagrammed wishing her good luck tonight