I have a 1996 b-3000 with a 3 litre engine and a bad altenator, there is a guy with a 4litre ford explorer with a good altenator for a good price, does anybody know if the altenators on a 4 and 3 litre are compatible?

Check out the specifics but the odds are against it. For $20, you can likely rebuild your current alternator--new bearings and brushes. It's actually fun to do.

The link below will give you a little perspective on rebuilding alternators:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-6.html#post666293

The typical Ford alternator is even easier.