This is what John and I were speaking of, the WHAT SIZE question and you can get 100 different answers. There are a lot of choices, I tend to go by the type and weight of the plane. There are 6 of us flying the Aeroworks Extra and all but myself are using the same power system. Mine is from another plane and just a bit smaller. We get together and do one big order from Hobby King in China. I just bought the bigger motor and haven't installed it but from watching others fly there planes I know what planes I can use it in. I have a scratch build on the way and this will be the system, it's an old time free flight plane and will be converted to electric. For $30.00 I got the TurnigyTR 2217/16 and the Turnigy Plush 25 amp ESC plus a 11X3.8 prop. It uses the Turnigy 1.3 Lipo battery, 20-30C1300 mAh battery. Those are about $10.00 a piece. I like to always go a bit over powered and I think this system would be a good choice for the little 4*.
There is a web site at the bottom of the page called Wattflyer and well worth looking at and reading the sticky basic. There is no support from HK but so far I have had great luck with them. I almost never buy anything that is on back order though. The one time I did it worked out well for me though but there are a lot of horror stories about HK but I have had good luck with them. $30.00 for a complete system is worth the risk.