4 1/2 is the creased height
a 4 1/2" open crown would be a bit silly because you would bash it and end up with something like a 3 1/2" crown
with open crown hats you could make the crown as low as you want really ... with the proper bashing
even a 6" crown you could get away with anything from 5 1/2 to probably 4"
anything lower than 4" ... to me at least ... is getting into a territory where you may want to question if you actually want a fedora or not ... your interest may be in a different style hat
Ah ha, that makes sense! So open crowns are built higher so you have the flexibility to lower them as much as you'd like when you bash. I am becoming more enlightened by the day.