On September 21, 1998, approximately 1700 Pacific daylight time, an experimental Duncan DX4, N4DX, impacted the terrain just after the pilot initiated the turn from downwind to base at Priest River Municipal Airport, Priest River, Idaho. The private pilot, who was the sole occupant, received serious injuries, and the aircraft, which was owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which was being conducted in the VFR traffic pattern, had been initiated less than five minutes prior to the accident. No flight plan had been filed, and there was no report of an ELT activation. According to the FAA Inspector who interviewed the pilot, while on downwind he thought that the aircraft was not responding to his lateral control inputs in the manner he expected it to. He therefore decided to continue around the pattern and make a full-stop landing. Because it seemed to the pilot that a reduction in engine power positively effected the perceived control problem, he reduced power on downwind and then attempted to turn base. Witnesses said that just prior to the base turn, the aircraft appeared to be going very slow. They also reported that as the aircraft entered a left bank, it appeared to fall out of the sky. Just prior to impacting the ground, the aircraft rolled back to a near wings-level position. According to the pilot/owner, no problem with the lateral flight control system was able to be identified, and no reason for the perceived lateral control anomaly was determined.