I think Alan meant a 3 7/8" hole-saw, that is slightly less than 100mm.
3 7/8" works out to 98.4mm, just about perfect for 100mm, even with a little runout.
I'd cut a practice hole in a piece of thin plywood and measure for gauge fit.
3 3/8"(about 86mm) is going to require a LOT of Dremel work, and make a LOT of dust!