For engine choice, taking into consideration value and performance, I would recommend either an ST75 or OS65AX. Both would give you plenty of power for the 4* but not "overkill." You won't be disappointed with either choice.
You will certainly pay less for a 2-stroke and get more power, however, 4-strokes have their own advantages (torque, sound, etc...) that make them worthwhile to many in this hobby. A somewhat cheaper 4-stroke alternative for your plane would be a Thunder Tiger F91S...they seem to be decent engines for the price. With the Tower discount you should be able to pick one up for right around $200...