No one has mentioned this yet, but with students that are ready to progress I always suggest they
INCREASE the THROWS on their existing plane to make it more responsive...you'll be surprised at how differently a plane flies with more elevator, aileron, and rudder responsiveness!
If you have a
computer radio start exploring the functions that it offers...add in a little
expo with those increased throws, try out some different mixing combinations to get your plane to fly a little better, start using your rudder to coordinate your turns. You'll notice that if you start coordinating your turns on a trainer you'll have to add less power on the turn to final while landing. Plus, you will amaze your friends...
With some direction, you can still learn a
LOT from your trainer, so after you get a second plane, keep your trainer around for flying (if you can afford to flight packs...RX, servos, battery, switch) so you can continue to benefit from your trainer long after you are flying that more aerobatic plane with the 4 stroke.
Heck, you could even
strap that 4 stroke on your trainer to break it in while you are waiting to get your plane, that way it will be ready to go when you finish building your second plane, and you will be confident in your engine.
Kudos to you for picking a 4 stroke...you may never buy a 2 stroke again, the four strokes are great engines and the most reliable I've ever had, and they sip fuel! All of the plane recommendations are great second planes! If you would like to build your second plane, don't hesitate to build while you
keep progressing on your trainer. So much can be learned from building a kit rather than just assembling an ARF. Once you've built a plane you won't be afraid to make small modifications, like shortening the wingspan, as someone else has done on thier 4*...
The most important thing...always think about what you're going to do on your flight before you take off. Have a plan, have lots of fun, and learn something from EVERY flight. You will be extremely surprised at how fast you get to be an accomplished pilot by focused practice.