Jazzy, I have the same M4s as you on my 1979 Koa SSB and comparing the measurements with the Schaller website it seems that they went from a 14mm “nut” on the collar to a 15mm. The string posts seem to be of a larger diameter too—probably a good thing. If everything else lines up it looks like widening the holes slightly would be your only choice. Seems a small (and invisible) price to pay for making the bass work well again. Just my $0.02 …
I am wondering if you would notice a difference with the new and lighter tuning machines in how the bass balances. Do you know the weight difference between old and new?
Cheers, Oliver