4 is there... I was pressed for time and was going to make the hybrids 5, but screwed up the numbering.
While I can see the appeal of making summons pets, I'm not sure I'd like to see weak summons. Or summons that eat up all your MP in 30 seconds. Or summons that say for a minute and then go away for 15. That's why I'd like to see a hybrid of 2 & 3 or 1 & 3. Don't forget that summons have to be balanced against other classes, so if squenix goes 3, the summons we are getting are diet coke of summoning; not quite summons enough.
Remember, in the new school systems, these summons replaced an entire party. So we aren't going to get 3 like we saw in the console games, and I'm not fond of that.