I have built four 4* 60's and highly recommend a 91 four stroke. I have put both OS 91's and Magnum 91's, the only difference is price, the magnum is about $100 less. I am not a speedster and the plane flys well at 2/3 power, I would however not go with anything less. You will really enjoy this combo, it is a very easy plane to fly and lands as slow as you want. Some people complain that is has a tendany to float. I put a 14X6 prop and this slow it down nicely.
Good Luck.