That would be my guess too, but have you done the diagnostic yet? (see my sig at the bottom of this post)
The reg/rectifier is very easy to change, it's the finned object on the left side of you bike, just below the seat. Also, you don't have to use the one for the B4, there are tons of others you can get that are cheaper.
See here:
http://forums.banditalley.net/viewtopic.php?t=2305Dita's been using a CB400 RR for the last 2 years, no hiccups, but read through the posts, there are tons of possibilities.
The service manual can be found in the Members section.