President Reagan was reported by a senior administration official to have listed for Prime Minister Nakasone four areas where the United States is highly competitive but feels it is denied full access to Japanese markets.

These sectors form the basis of the new Reagan-Nakasone market opening approach, being supervised by Secretary of State George P. Shultz and Foreign Minister Shintaro Abe.

In addition, Reagan administration officials are concerned that a Ministry of International Trade and Industry (MITI) proposal would create "an enormous barrier" to the sale of American computer software in Japan.

The four sectors that President Reagan targeted for market-opening moves are:

* Telecommunications. Legislation establishing competition in the Japanese telecommunications industry and turning the government-controlled telecommunications monopoly, Nippon Telegraph and Telephone (NTT) into a private company has the potential of opening the huge Japanese market for sophisticated telecommunications equipment to U.S. manufacturers.

Because the details surrounding this vast change in Japan's telecommunications system are being worked out behind closed doors, American companies and the Reagan administration fear the equivalent of an "old boy network" of Japanese suppliers will erect subtle barriers to the purchase of American products.

The Nakasone government insists that American companies will have equal access. But foreign suppliers are not members of key Japanese committees setting specifications and deciding methods of certifying products for use in Japan. This has aroused suspicions that specifications will be designed to exclude American products and certification will be hard for foreign companies to obtain.

These are all methods that Japan frequently has been accused of using to protect other markets.

The telecommunications area is extremely sensitive because Japan took advantage of the court-ordered breakup of American Telephone and Telegraph Co. to flood the United States with telephones and telecommunications equipment.

Japan's new teleommunications law, which passed the Diet (parliament) last month and which takes effect on April 1, is seen as the Japanese equivalent of the breakup of AT&T.

* Forest products. During an election swing in the Pacific Northwest last year, President Reagan promised to press Japan to buy more American products made from cut timber. These include plywood and paper board.

Japan prefers to buy whole logs and make the products itself, giving jobs to Japanese instead of American workers.

This is a sensitive area for Japan because its forest products industry is in a recession.

But Reagan administration officials feel that American products, which may be less expensive than domestic goods, should be allowed equal access.

* Computers. Because it is racing to catch up with the United States in sophisticated computers, Japan has been reluctant to buy American-made products, the Reagan administration believes.

Officials cite cases where Japanese universities have said there is no money in the budget to buy American supercomputers, but the funds become available as soon as a made-in-Japan model comes on the market.

This denies American companies access to a lucrative market in which they have a competitive advantage, while Japanese companies have full run in the United States.

* Drugs and medical equipment. Japanese health officials have erected nontariff barriers to American drugs and sophisticated medical equipment by insisting that clinical trials on foreigners are not acceptable in Japan.

In other areas of controversy:

* MITI still is pushing its software proposal, although American lobbying kept it from being brought up to the Diet last year.

Japanese officials call it a turf battle between government agencies with MITI trying to get control of software licensing.

American trade officials, however, are irked that they had to spend a year fighting a potential new trade barrier instead of peeling back old ones.

* The deadline on the Nakasone government's pledge to liberalize Japan's financial markets is April 1, and there is some concern in the Treasury Department that Japan is moving too slowly to meet it. A senior administration official does not hold that view, however.

* Finally, four years of Japan's so-called voluntary restraints on auto sales in the United States end March 31. The Reagan administration and the American auto industry are divided on whether to ask for another year. It is also a knotty issue for the Japanese government, which fears that unrestrained exports of Japanese cars to the United States will capture so much of the U.S. market that they will unleash massive protectionist measures.

That subject, by mutual agreement, never came up at the Nakasone-Reagan talks.