That's the beauty of the Canon EOS Rebel series...the lenses are interchangeable. I just upgraded to the T4i from the T2i and even the battery grip/remote shutter switches/and battery are interchangeable.
Your T3 lens will function just fine on a T4 body (and visa versa). Both bodies are offered with several kit lens, so the lens in a particular kit may or may not be identical to the one you already have.
This camera accepts all Canon EF and EF-S lenses and will probably accept other manufacturers' lenses that claim compatibility with Canon DSLRs (e.g. Sigma and Tamron). I use multiple EF and EF-S lenses and a Sigma with my T4i.
I'm using Sigma lens on my Rebel T4i. I bought them for my Canon Rebel XT years ago. These lens have Canon mounts on them. I didn,t get enought time with this camera to see if it is as good as the older Rebel