President Carter is expected to ask Congress to construct four enrichment plants that would produce uranium for electricity by a new method that has been secretly developed for almost 20 years at the Oak Ridge National Laboratory in Tennessee.

Administration sources said the President will announce in his energy policy message today his wish to build four gas centrifuge plants that would cost an estimated $4.4 billion and produce, over 30 years, enough enriched uranium to run 90 atomic power plants of 1 million kilowatts each during that period.

To be built over the next 10 years at locations still not determined, the centrifuge plants would serve to back up Carter's promise that the United States will continue uranium needs in exchange for other nations' waiving their rights to use the plutonium fuel generated in the burned-out uranium.

The indefinite postponement of plutonium use is at the heart of Carter's policy to prevent the sapread of nuclear weapons. The uranium that the United States supplies the world from its three enrichment plants cannot be used to make atomic bombs. THe plutonium that can be estracted from the burned-out uranium can be used on weapons.

The centrifuge method of uranium all of its development has been classified, since the centrifuge technique could be tailored to make uranium for weapons as well as electricity.

Since World War II, uranium has been enriched in the United States at factories in Oak Ridge, Paducah, Ky. and Portsmouth, Ohio by a techique called gas diffusion. The same technique is used in the Soviet Union, France and Great Britain, the only other countries of the world with known enrichment plants.

Gas diffusion involves pumping uranium hexafluoride gas through thousands of membrane-like barriers, which trap the heavier isotopes of uranium called U-238 and allow the lighter isotopes called U-235 to pass through. The heavier isotopes do not fission and cannot produce electricity. The lighter ones fission and produce electricity.

The centrifuge method involves the same hexafluoride gas but spins the gas in supertough steal cylinders that bring the lighter isotopes to the top and settle the heavier ones on the bottom, just like the centrifuges at a dairy separate milk from cream.

The advantage of a centrifuge plant is that it costs much less to operate than a diffusion plant. Centrifuge uses less than 10 per cent of the electricity diffusion does, which would mean a saving of hundreds of millions of dollars over a year.