some will, some won't, some do, some don't, some might.
I have a No1 that some no 4 mags won't fit, too tight at the front of the mag well. tiny bit of judicious filing with a needle file in the front corners relieved that problem. three or four strokes tops. some people experience feeding problems, mags falling out, i myself have not.
all my No 1 mags fit both the No 4 and No 5