I think that main trick here is - how fast LTE will be implemented in countries that Apple consider big marketplaces.
If their management saw potential in fast growing LTE, than I am sure iPad 3 will be LTE supported. If they dont see potential, than iPad 4 is reserved for LTE.
Be aware that Apple built his highly respected image in tech. world because he is one that brings fresh technology to customers, that others just cant in that moment.
I agree it could be a big selling point for many. I would just hate for people to think if they buy an iPad 3 thinking they will get a fantastic new level of networking without realising only a few places and operators currently run an LTE/4G service.