A #4 in good shape is about wide open as to calibre. The only thing to actually think about, is the feeding and will the rim fit through the tiny hole of the action rear. This eliminates anything larger than the .348 or .45/70 case - about identical rim size.

Depends on what you want - for calibre or power.

Many .308's and .223's have been built on #4's & no modification to the extractor - imagine!! I figure the smallest case the extractor will function with, is the .22 Hornet.

I have a #4 sporter - poorly designed & needing refinishing montecarlo stock and sporter forewood. Brand new .303 2 groove barrel of large proportions (.314" groove), so I ran a .300 Winchester chambering reamer in 2", then cut a belt groove for headspacing. I then necked down .350 Remington Mag brass to take a .312" bullet. What I have, I found out, is Ken Waters .312 Express. I am running 174gr. Hornady RN's out at 2,960fps (Re#19 or maybe Re#22). not too shabby and it shoots 1 1/2" for 3 shot groups @ 100 meters using the original battle sights. Did this many years ago, haven't shot anything with it yet. I expect whatever it is will die.

I was going to rebarrel it to .45/90. Thought of a .223, even, or Tac .20 or .204 - nice 1 1/2 pound trigger - but no, it's a clunker for big game - might make up a great .35 on an improved .303 case - about .358 Winchester ballistics - could go normal .358 Winchester, could go .356 Winchester - could make up a nice .375/303IMP - just about anything is possible, within reason. Would even make up a nice .416/.303IMP - tiny shoulder but that's OK. Normal taper including the neck will make up a .40 cal. straight case, like the original BP .40 Maynard.