RE: prop 55ax at 4500 feet
I usually run a 12 x 5 on mine at 5000 feet and it works great. Play around with a few. Keep in mind these engines need 20% oil with some of it castor. Make sure the engine is well broken in, which can take a while on these. I would run 15% nitro at your altitude and an OS8 plug as well.
Great engine, I love all of mine, the stock ones and the modified ones.
Good Luck
Scot