Given the distances involved in airsoft, a scope is not needed. It comes in handy once in a while and makes your gun look cool. But do you
need one? Nope.
You have the right idea for a 4x scope, anything more than that is just silly and unnecessary. You can get a decent quality ACOG replica, huang ACOGs are pretty good. I have one and its got decent eye relief and the clarity is pretty good. If you want a more traditional scope you can go for a Bushnell for the lower end of the price spectrum and still retain good quality.
http://cgi.ebay.com/bushnell-sportsm...#ht_1208wt_958
Tasco makes some good scopes as well, same with Nikon, Pentax, Leapers, but the nicest, clearest and sturdiest scope I have seen is a Leopold. Those bastards are pricey though.