I personally think that while it'll have a largish following, It wont get itself anywhere near as large as G4. I think that if the show has a good quality, not even to the level of FIM, that there would be around the same number of new bronies entering the fandom as we have leaving, leading to a null zero situation.
Now if the show is able to be as good as the one we have now, then we have a different kind of beast on our hands. I could see a large increase in our fandom's membership, but I dont see it ever reaching it's hight again.