There was a post on EqD a few days ago. It was about the new Equestria Girls movie and the Rainbow toys. They also said something about how much can we see from Gen4. Take a look:
Webcast is over and an executive said the following, that MLP will last five more years. If we assume one season per year we can expect at least five more seasons. To clarify this is only an assumption! As mentioned by others the current generation could last five years but not the show.
What do you think? We get 5 more seasons? Less? Tell me what you think! Also, here is the link if you don't belive it: http://www.equestriadaily.com/2014/02/hasbro-investor-webcast-information-eqg.html