A radical enhancement of four-wave mixing in hollow-core photonic-crystal fibers is experimentally demonstrated. An enhancement ratio of about 800 relative to the regime of tight focusing is achieved for the four-wave mixing process 3ω = 2ω + 2ω -ω, where ω and 2ω are the frequencies of fundamental radiation and the second harmonic of picosecond Nd:YAG laser pulses.