Charles W. Shuman, chief of the Farmers Home Administration for the last 3 1/2 years, announced his resignation yesterday.

Shuman becomes the fourth high-level Agriculture Department official to resign as the Reagan administration gets ready to debate major farm legislation in the coming session of Congress.

Shuman came under fire when the administration began tightening up on loans to farmers and foreclosing on many of them. The FmHA and Shuman also were the subjects of criticism when the agency spent $646,000 to develop a complicated 26-page financial disclosure form for farmers.

Shuman told the Associated Press that his resignation, effective Jan. 31, was for "strictly personal reasons" and not as a result of any pressure brought against him or the agency.

Shuman became head of the FmHA in August 1981. Before that, he was a stock and commodity trader in Illinois, and a partner with two brothers in a farming operation in Sullivan, Ill. He was state director of FmHA operations in Illinois from 1971 to 1977.

Although Shuman declined to reveal his plans, he said that he is looking at a number of "nongovernment opportunities."

In addition to Shuman, Richard E. Lyng, deputy secretary of agriculture; William Lesher, assistant secretary for economics, and John B. Crowell Jr., assistant secretary for natural resources and environment, have announced their resignations.