On October 19, 2013, about 1600 central daylight time, an experimental amateur-built Pohoski RV4 airplane, N10XP, landed off-field near New Waverly, Texas, when the pilot smelled smoke in the cockpit. The pilot was uninjured. The airplane was destroyed during a subsequent ground fire. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Day visual flight rules (VFR) conditions prevailed for the flight, which did not operate on a VFR flight rules flight plan. The flight originated from the Livingston Municipal Airport (00R), near Livingston, Texas, about 1540, and was destined for the Sandy Creek Airstrip, near New Waverly, Texas. According to the pilot, the airplane was refueled at 00R at 1528. About four miles from the intended destination, he smelled and saw smoke and turned toward Estates Airpark, which was the closest airport. The smoke increased in intensity so the pilot retarded the throttle to idle. He executed a precautionary landing in a pasture north of the diversion airpark. The airplane was intact when it came to rest and the pilot saw smoke and flames as he exited the airplane. The pilot reported that the airplane burned and that a patch of grass, about a 15-foot diameter, was burned around the aircraft. The pilot indicated the smoke and fire was coming from the tunnel (at the firewall, between the rudder pedals) that is between your feet, in the RV-4. He stated that the fuel selector was mounted as shown on the kit manufacturer's plans for the tunnel. The selector valve's out-port was connected to a fuel filter, the filter was connected to an electric fuel pump, and the pump was connected to a bulkhead fitting on the firewall. The tunnel additionally housed the forward stick assembly and a conduit for wires from the back of the aircraft. The pilot said that the stainless steel firewall remained intact, however the wires and controls which ran through fittings did burn. The battery was mounted in the baggage compartment and not in the tunnel as shown on the plans for center of gravity consideration. The pilot stated that there was absolutely no seeping fuel; no telltale blue stains or fuel smell in the cockpit. He said that he did a "lot" of aerobatics, and in positive or negative g flight there was no smell of fuel. The pilot stated that the smoke he smelled was not electrical smoke but combustion smoke. He did not have time to look under the panel or troubleshoot the smoke and landed immediately. After landing, the pilot did see flames at the firewall side of the tunnel as he exited the airplane. He said, "What caused it I do not know." The pilot said that the first item he checked on a preflight is fuel or oil under the airplane. The engine was always leak free and the floor was dry when he took the airplane out before the accident flight.