Took the boat out for the first time this year, tried out the 21" Mirage Plus I had picked up last year to compare to my Revolution 4 that was on the boat when I bought it.

The Rev 4 is a far superior prop in my application...hands down. Hole shot was twice as fast with the Rev 4. The trim response is much more 'linear' and with a larger range before blow-out. Holding turns with the Rev 4 is effortless, rarely needing to trim down, the exact opposite was true for the Mirage. The Mirage didn't seem to be as efficient, either...my Rev 4 runs about 36 mph @ 3400 rpm (which is where I like to cruise with the bimini up), the Mirage was right at 34 mph (GPS measured)
SO...all in all, a good experiment and I will keep it on board as a back-up.

Are they both 21pitch?

I'm curious how a Bravo would stack up against the Rev 4 with both being 4 blades?

Would like to know also, are they both 21P ?

Sorry, yes...both 21" pitch. My other boats I have had responded really well to a Mirage Plus, my last Baja had a lab-finished 21" that was simply the perfect prop for that boat.
I have owned this 252 for three years now, and was impressed with the effortless and near-instant coming-on-plane it does. So I learned a lot here.

A Rev 4 21P = Mirage 23 so not the same
Ignore analysis so far as I'm concerned