I've also been using a PIAA H4 bulb in my R65, but it has lasted several years so far with no signs of problems. IT is definitely whiter and brighter than the old bulb that was in the bike when I got it. I don't have the part number handy though.
I would certainly recommend using a new, whiter/brighter bulb (within reason) if yours is not a recent vintage H4 headlight bulb. Don't try to get a super high wattage one though, as you may have other electrical issues to contend with. Make sure to use a good relay for high powered or auxiliary headlights (and don't run them when you are just putting around town, unless you also install a high output alternator)