I mean, yeah that could happen: Eclipsa was never mentioned before Season 2 but seems to be pretty important now, though people knew there would be an important character with spades because of Miss Heinous before Season 1 was over, so even that doesn't entirely count.
I just don't think they're going to end all existing plots to do it, just build on them the way they did this season.
Because if they do really want to end all the major plots in 3 and then do something totally different in 4, it pretty much means Season 3's handling of those plots will have to be completely perfect 10/10 total masterpiece for Daron to NOT look like a hack. Because if Season 3 and 4, in this scenario, are any less than perfect, fans will wonder why they couldn't have used 4 to devote more time to the major plots instead of wrapping them all up quickly and sloppily.
It doesn't mean that the plots will necessarily be handled better if continued across all four seasons, but it does mean they'll have more time to develop and more chances to be done well.