Going by how MLP reboots tend to go, I’d assume something like what Megas75 is proposing – generally, at least a few characters make the jump to the new core cast when the franchise goes through a reboot, but not always all of them. That being said, previous main characters still tend to reappear, if only in secondary or minor roles.
Example: the G3 show had seven main characters – Pinkie Pie, Rainbow Dash, Scootaloo, Sweetie Belle, Toola-Roola, Starsong and Cheerilee. Pinkie and RD, obviously enough, made it into the main G4 cast. Scootaloo and Sweetie Belle may not be part of the mane 6, but they’re among the show’s main characters all the same. Cheerilee returned as a supporting character, and Toola-Roola had a one-off appearance in Season 7. Only Starsong, to my knowledge, didn’t make it into FiM in one way or another.
That being said, not all of these were direct carry-overs of characters – Scootaloo and Sweetie Belle’s roles and characters obviously changed quite a lot, while RD and Pinkie include a lot of character traits from the G1 characters Firefly and Surprise, and can be considered successors to them too in that sense.
And of course, Twilight Sparkle, Applejack and Fluttershy are reimaginings of G1 characters as well, Twilight and Applejack and Posey, some sharing more with their prior incarnations and some less.
Now, my actual point past all this rambling: MLP does very much have a cast of recurring characters who return in most iterations with more or fewer changes. That said, each generation’s take on a character can differ quite a lot from the others’, names can change or stay the same, and not every character is always reused. Thus, my expectation is that we’ll see most if not all of the G4 mane six in the new show, but I do not as of right now expect that they will all be among the central characters this time around. While it’s certainly possible they might, I think it’s quite likely we’ll see a couple of G4 supporting characters, or for that matter of characters from previous gens, reimagined to fit among the central characters.