DAVOS, SWITZERLAND -- Suddenly, there is a new focus on Japanese investments in the United States. Because of the slump in the dollar since 1985, Japanese (and other foreign) investors have been able to buy into American companies or acquire real estate and other assets at bargain-basement prices.

A surge of direct Japanese investment in industrial and other business operations has rocketed Japan into fourth place among the largest foreign investors in the United States, behind Canada, the Netherlands and Great Britain.

There has been a growing uneasiness about this trend, even within the free-market-oriented Reagan administration, the fear being that Japan will be the principal beneficiary of the new partnerships. In a notable case last year, Fujitsu was blocked from acquiring Fairchild's semiconductor division, even though it was in the hands of Schlumberger, a French firm.

The immediate trigger for the new debate is a surprising speech last week by the influential New York investment banker Felix Rohatyn. He worries that Japan is now in a position to buy controlling interests in major companies and banks, because the yen is so strong and the dollar so weak that American assets are undervalued.

He cited as an example Fuji Bank, which is worth a staggering $60 billion, 10 times the $6 billion value of Morgan Guaranty. Rohatyn suggested the United States consider setting up a ''review'' process that could reject certain foreign buyouts as against the national interest.

At the World Economic Forum here bringing together leading businessmen and officials, a Reagan administration representative told me privately that with increased Japanese ownership, the risk is that policy decisions will shift to Tokyo. ''Then we will become merely the day-to-day managers,'' he said. ''We don't want the situation to degenerate into 'their minds and our muscle.' "

But an increased flow of Japanese money into the United States is not only an inevitable result of the decline of the dollar, but a practical response to the threat of protectionist legislation. Moreover, the creation of joint ventures, blending Japanese manufacturing skills with American technological innovation, holds the promise of strengthening America's industrial base and promoting a recovery -- for the United States -- of export market shares.

But suppose the dollar plunges even farther, making takeovers still more attractive for foreigners? According to a presentation here by Stanford University professor James Howell, some Japanese officials say privately that a rate of 80 to 90 yen to the dollar could be tolerated by certain Japanese industries.

That view was sharply challenged in an interview with the influential Japanese vice minister for international trade and industry, Makoto Kuroda. He thinks that the yen is already overvalued.

Kuroda sees increased Japanese investment in the United States as a natural development in the integration of the two economies, a development that is likely to continue if there is stability in exchange rates around current levels. A further sharp drop in the dollar poses a capital loss for Japanese investors, Kuroda said.

Rohatyn recognized from the start that his suggestion would be highly controversial. For the most part, businessmen and bankers to whom I talked here see his suggestion as a hysterical response to a new global situation, in which interdependence will increasingly be reflected in mixed multinational ownership and production.

American resistance to foreign investment is analogous to the European fear in the 1960s that a then-powerful America would gobble up the best European companies. That concern was popularized by Jean-Jacques Servan-Schreiber's ''The American Challenge.''

But American investment strengthened Europe and its economy, just as Japanese investments are likely to prop up many an American institution. For years, the United States has criticized the Canadian investment-review process, arguing that it insulated Canada from the growth benefits that could be achieved with the help of outside money.

What really bothers Rohatyn is that the United States has become overwhelmingly dependent on the influx of foreign capital to finance our huge budget deficit. That, he fears, makes the United States vulnerable to political decisions in Japan, West Germany and other nations now in surplus. These nations, friendly to us today, may have reason to shift their views later. Rohatyn recalled de Gaulle's observation that sovereign nations have ''no friends, only interests.''

But America's excessive dependence on loans from its allies to cover the budget deficit has to be attacked by a shift in macroeconomic policies, notably a willingness to raise taxes in the United States, and a European willingness (especially in West Germany) to fight an expected recession this year with expansionary policies. A hasty cutoff of direct foreign investment would only delay the refurbishing of America's industrial base.