Hmm. It would be different story if you didn't have either one yet, and you were deciding on which to buy. As you already have the T4i, which must have been a very recent purchase, I just don't see trading it right now for a used 3-year old camera that is about to be replaced/refreshed with a newer model. The 7D does not have a better sensor, and the T4i actually has a pretty nice new AF system itself, with all the cross-type AF points.

Unless you were doing extreme outdoor photography and really needed weather sealing and a metal body, I just don't see a huge difference there for you. Plus, with all the new stuff just about to come out, you might rather grab the 7D mk2 if it has a much improved sensor, or maybe add a full frame 6D or something.

But more than anything, I agree with Busted that you should probably first be sure you have good glass.