KANSAS CITY, Mo., May 1— Farmers and grain traders here in the Middle West, like the Washington agricultural advisers whom many of them heed, were reacting with restrained enthusiasm today to the Soviet Union's agreement on Thursday to buy four million metric tons of American wheat.
The reason for the restraint was that the amount involved, a total of 147 million bushels, represented only about 8 percent of a surplus totaling 1.88 billion bushels that is expected to be in storage in midsummer when a new harvest of about two billion bushels will be pouring in.
But the enthusiasm reflected the belief that the agreement was a clue to a renewed commitment by the Reagan Administration to wage a tough fight with trade competitors, using some of the toughest features of the current farm law to meet foreign prices and subsidize exports. It is also widely viewed as a sign that other deals will probably follow.
Carolyn Rains, who shares the management of about 2,500 acres of wheat land in Wellington, Kan., summed up the implications of the development. Growth in Global Production
''It's a start,'' she said. ''It's by no means an immediate answer to our wheat problem; not this sale, anyway. But if it continues it could certainly help our trade and our farm problems too.''
The development must be viewed against a background of global production that has overwhelmed demand. As a result, American surpluses have piled up and farmers have suffered.
The Food Security Act of 1985, along with a new Export Enhancement Program, offered new devices to deal with the problem. Under the 1985 law, the Government could reduce price supports sharply in an effort to make American farm prices more competitive and to make up the losses to farmers with income subsidies. And where that device failed to push prices low enough to make deals, the Export Enhancement Program could take over, offering subsidies to promote foreign sales.
There is some indication, as Martin Abel, a Washington consultant noted today, that the threat posed by those programs has already had some effect. It appears to have led some rivals, including Australia and Argentina, to cut back on some of their grain plantings.
But until recently the Reagan Administration refused to extend the subsidy benefits to the Soviet Union that it offered to other trading partners. Need for Federal Approval
The reason for any remaining uncertainty is that any deals must be made between the Soviet traders and private American sellers and then submitted to the Government for approval of the rate of subsidy involved, as John Schnittker, another Washington consultant, noted. Under current conditions a deal might work like this:
The Russians would buy a portion of their wheat from a trader at the world price of $82.50 a ton. But the dealer would have to acquire the grain at American prices, now about $115 a ton at export terminals.
The shipper would then submit the deal to the Government for approval of a subsidy to represent the $32.50 difference. The announcement of the agreement on Thursday has led American traders to assume that reasonable deals will gain approval.
''One has to assume that the Russians and the American Government are dealing in good faith on both sides,'' one prominent trade official said.
Rumors about an agreeement had already raised wheat prices about 30 cents a bushel, even before the deal was closed. Wheat Prices Rise
Today the wheat market at the Kansas City Board of Trade reacted with continued but restrained enthusiasm. Contracts for July delivery of wheat rose 6.25 cents a bushel in heavy trading.
One reason for the optimism, according to agricultural experts, is that both farmers and traders now assume that this is only the first of an expanding series of deals that they expect to be made with the Russians. And behind assumption, some say, is more than a Soviet wish to meet the provisions of a five-year trade agreement signed in 1983. That deal called for the Russians to buy at least nine million tons of grain a year, including four million tons of wheat.
In the last two years the Russians have bought very little, saying that American prices were too high.
Some experts suspect that the Russians may be motivated to buy more wheat now out of a desire to assure themselves of an American source because of a recent deterioration of the Soviet grain crop. The Soviet wheat crop is now estimated at 195 million tons, down from last year's harvest of 210 million.
Major Russian wheat regions suffered a harsh winter, and severe damage to winter wheat is suspected. In addition, weather problems delayed the planting of spring wheat, exposing it to severe heat and drought problems that can afflict late-maturing crops.
Any further deterioration, experts say, could lead to still more sales of wheat to Russians. And the new willingness of the Reagan Administration to use its subsidy tool, they say, enhances prospects that Soviet traders may fulfill even more of the provisions of the five-year agreement and buy other grains, such as corn and soybeans.