It seems to me that the bride is in charge here. If she is happy with you shooting a few 4x5 shots, then that is fine. A contraindication is if there is a contract stipulating that the professional photographer is the only photographer allowed.

Also, it would certainly be a nice courtesy to see how the professional photographer feels about the matter, even if he/she has not been granted exclusive right to photograph. After all, you would not want the pro to be mad at you and the bride because there is just an outside chance that the anger could show up in the product somehow... not that it necessarily would mind you, but I'm just sayin'...