For reference, there is a bi-xenon kit for H4 applications; my BMW mechanic keeps trying to sell me kits for both the BMW and the Monster (very tempting). AFAICT you wouldn't lose the ability to flash, as the H4 bi-xenon setups have this trixie deal where the bulb itself moves from one position to the other.are you talking about the real HID with ballast or just an "HID look" bulb? Two VERY different things you know...
With real HID kit, the problem is that our bulb is for both high and low beams. Unless there's already a bi-xenon kit out there for H4 application, your true HID will only provide low beam and you will not have high beam. But even if bi-xenon kit exists, you still will not be able to flash your high beam when using low beam
One of these days when I'm feeling spendy I'll put HIDs in one or both bikes; they really do make a tremendous difference (I have them in both of my cars and can't hardly go without anymore).