Frank ....your making it hard for yourself here, .A#4 barrel has the same thread as a #1 so it will screw straight on, but....the barrel timing is different so the extractor slot will be on the wrong side,so ...all you need to do is machine half a thread+ or- off the breach and the same amount off the shoulder,recut the extractor slot, tidy up the chamber and Bob's your uncle.

cheers
Pete