The bulbs on a 600 are H4 high beam and H1 low beam, the 400 is 2 H4's. On the 400 the both bulbs are on for low and high unlike the 600which has H1 for low beam and H4 for high beam and only the high beam filament works. The reflectors on the 600 are also only low/high beam for each side/bulb. Even when the 600 is modded so both are on, its still a big trade off as the high beam reflector has to be lowered to stop dazzling on coming, which then means loss of high beam. The 400 has dual reflectors and all filaments on the bulbs work.
Quite why the 600 has such a piss poor set up is beyond me just penny pinching perhaps.