One the Air Force`s best-known weapons designers, who is a key paid adviser to one of the most prominent congressional critics of the Defense Department, is suspected of triggering an explosion of an anti-tank shell in a suburban gasoline station that severely injured four people.

Col. Robert Dilger, 54, of Xenia, Ohio, who as an Air Force officer designed the anti-tank weapons of the A-10 jet attack bomber and cockpit equipment of the F-16, was arrested Tuesday after a 30 mm. armor-piercing shell was fired from unmounted anti-tank gun in the back of his pick-up truck. Dilger, who retired from the Air Force in 1980, was paid $100 a month as an adviser to Rep. Denny Smith (R., Ore.), the chairman of the Congressional Military Reform Caucus, and assisted the congressman on military affairs, Smith`s press aide confirmed Wednesday.

The explosion occurred at Texaco station near the Pentagon in Arlington County, a suburban community in Virginia. Dilger and Joseph Donahue, 40, of Killeen, Tex., were seen examining the weapon the back of Dilger`s truck moments before the explosion, according to police accounts.

The shell passed through the side of the truck and hit a gasoline pump, causing a 20-foot geyser of burning gasoline, according to police accounts.

Four people were injured in the blast, including a 64-year-old man who was in stable condition after treatment for a large shrapnel wound in his leg and a 60-year-old woman who received 60 cuts from flying metal.

Arlington police said that both Dilger and Donahue attempted to flee the scene of explosion but were arrested nearby. They are under investigation on criminal charges in Arlington and for federal weapons charges that could arise from the possession and firing of the anti-tank cannon. According to a spokesman for the federal Bureau of Alcohol, Tobacco and Firearms, agents also found 18 unfired shells in the truck.

The case is also under investigation by the FBI.

The two men were charged with possession of an explosive device and released on $1,000 bond.

It was unclear why Dilger had the weapon or what its purpose might be. He is a private citizen with no clear authority to possess or work on such a weapon for the military forces, according to a Pentagon spokesman. Greg Walden, a press aide to Smith, said Dilger was working on the design of a new anti-tank weapon and the accident might have been connected to this work.

A police spokesman in Virginia said that the weapon was not registered with any federal or local agency and that it was virtually impossible for anyone to legally possess such a weapon in Virginia.

Both Dilger and Donahue had been informally involved in the operations of a group on Capitol Hill called ''The Foundation,'' which did research work on defense matters. Smith and Sen. Charles Grassley (R., Ia.), strong critics of the Defense Department, are honorary officials of ''The Foundation.''

Donahue described himself to Capitol Hill aides as a former helicopter pilot, and he and several other members of a group of critics of the military also delved into counter-insurgency, guerrilla warfare and counter-terrorism. The anti-tank gun, effectively a small cannon, would be very valuable to terrorists, and 30mm shells of the type found are capable of piercing the armor of small tanks, armored limosines of the type used by President Reagan and other officials and of bank armored cars. The shell could have wreaked extensive havoc if it had continued on its path toward homes near the gasoline station.

Dilger was a decorated fighter pilot who had flown dozens of missions over Hanoi during the Vietnam War. Smith, who is also a former Air Force pilot, flew with Dilger in Vietnam, according to his press aide. In a statement issued by his office, Smith said he was ''deeply concerned by the tragic accident'' and said he doubted that Dilger could be involved in anything improper.

After the war, Dilger was assigned to designing weapons and equipment for fighters and bombers at Wright-Patterson Air Force Base near Dayton, Ohio. He was credited with making major contributions to the cockpit design of the F-16, one of the Air Force`s supersonic interceptors, and with adapting a 20mm cannon in the A-10 fighter-bomber so it could destroy tanks.

After he left the Air Force, he became an outspoken critic of lack of military preparedness. Neither he nor Donahue could be reached for comment.