I used a 4 1/4" Dave Brown on mine when I had it.

the spinner was cut for a 3 blade Mezlik 24x12 and fit perfectly. the back plate was the correct size for the cowl at 4 1/4", I think 4 1/2 might be a bit big, not sure.

I don't concearn myself with getting the "perfect" fit to the cowl. once you fly the plane you may need to make thrust angle corrections during trimming so a little gap is needed.

Hubb