It's a line voltage spike that did a job on your low beam filaments -- the glob of molten tungsten at the end of the filament support is the giveaway. Sounds like your voltage regulation is not what it should be. Are you running relays in a setup anything like
this? Or just running your H4 conversions on the stock wiring?
Those are Philips Xtreme Power H4s -- the best standard-wattage H4 bulb made. But they, like any other bulb, will not withstand a high line voltage spike.