white wire from the alternator - whats its purpose, and will this prevent me from using the modern piece?
The white wire provides voltage to the alternator to energize the electromagnet.
It looks like your F4i has a permanent magnet which has no control. I don't expect the "new" one you've chosen will work with the SOHC4. Sorry.
Cheers,
In simpler terms, you cannot wire a reg/rect designed for a permanent magent alternator to a system having an electromagnet alternator, or visa-versa. There just isn't any way, because the regulator portion is activated in a totally different manner. The permanent magnet type regulator actually shorts the stator to control charge, while the electromagnet type regulator turns on and off the alternator's electromagnet. Very different systems. For reference, there are very few bikes made today with the older electromagnet design. Probably 95 percent are permanent magnet now.
P.S. Yes, the 900F one will work on an SOHC 750, but the color codes won't match even though both are Honda, because when Honda went solid state in the late 70s they moved the regulator from the positive side of the field coil to the negative side, hence the regulator has differing wire colors.
