If that happened, I'm cool with it. It would have ended in a very good spot, and as long as they don't fuck up the ending entirely, I'd be chill with it, and wait until Gen 5.
Of course, there would be one a-hole who freaks the hell out over the show ending, probably throw a huge temper tantrum and will take their place amoung the ranks of "Things that give the mlp fandom a really bad rep". But I doubt the show will be cancelled after seaon five, (They're making money off it, so,), leaving the question of who will be the major tantrum baby of the fandom, up for speculation. But I don't see anyone fitting the description for that yet.
Except maybe Chris-Chan.