On July 8, 1999, at 1615 Pacific daylight time, a homebuilt RV4, N9233A, registered to and operated by the second pilot as a 14 CFR Part 91 personal flight, initiated a forced landing to an open field shortly after takeoff from Hobby Field, Creswell, Oregon, after the engine experienced a loss of power. The aircraft landed hard and was substantially damaged. Visual meteorological conditions prevailed at the time and no flight plan was filed. The flight instructor and private pilot were not injured. In a written statement, the flight instructor reported that he was seated in the rear seat with the non-current private pilot in the front seat. The flight instructor stated that the private pilot was practicing touch-and-go landings. During the takeoff from the fourth landing, the flight instructor reported that the engine lost 80 to 90 percent power. The flight instructor stated that he took control of the aircraft and made a right turn to avoid power lines and landed the aircraft in a hay field located about 2,000 yards north of the airstrip. During the post-accident engine inspection, a small piece of the alternator belt was found ingested into the carburetor. The flight instructor reported that the carburetor heat was on at the time of the power loss.