It's a tough call... if you have access to the race prepped F4, rock that for a while and see if you can stand it. Otherwise, the piggy might be a good one to go track with. But building a decent RC to just beat around with on the track can't be that expensive. I know a good source for parts.
The worst part about track RCs is that if they go down, you're in for some damage... exhaust, rearsets, bodywork, handlebars, fairing stay. Some of the inline fours have more plentiful (read: "cheaper") parts if needs to be replaced. Oh yeah, and tanks... oh man, the ephing tank bulges.
I know there are some members on the board that have thrown together cheap-o track RCs and they are great. Goes down, scratched exhaust, dented midpipe? Whatever, just pick 'er up and roll on down the road. On the other hand, if you have a cherry rider you might not want to lay down, maybe tracking it isn't a great idea.
Keep the RC no matter what your track ride is!