T3i focusing better than T4i

Sounds like you've made your decision and are returning the T4i, so this may be moot. When the T4i first came out, I was interested in it as well, so I looked at all the initial impressions. I did see a direct comparison to the T3i that showed a ​little ​better IQ, under certain moderately lit subjects, but it wasn't eye-poppingly obvious. What the T4i offers over the T3i is of course the touchscreen, somewhat better AF for video with the right lens, some tweaks like in-camera CA correction, a screen-off proximity sensor, and a few other small things. All that weighed against the subtle difference in IQ. If your focus (no pun intended) is purely focus accuracy and all the pluses of the T4i are not something that you'd take advantage of, keep the T3i and a few $$. But sometimes a camera that's more fun or easier to get the shot taken is worth the compromise.