A few days back, Rockman Perfect Memories member, Hypershell, created a hack to incorporate voice clips of Lucas Gilbertson into the PC version of MegaMan X4. The result:
If by chance you own the PC version of MMX4, feel free to grab the hack here.
UPDATE: A second patch is now available, this one includes voice clips of Mark Gatha, Japanese opening and ending songs and a few glitch fixes. Grab it here.