TS4 isn't even finished yet. We don't have werewolves back, after all, plantsims need to be upgraded to a full lifestate, we could really benefit from a pack focused on superheroes, not to mention all the people clamoring for fairies. On the realism side, we don't have the instruments or ability to put a band together, we don't have cars, there's a certain subset of players who are foaming at the mouth to get babies out of the crib already, we need more activities and interactions for Children and Teens (and some may argue, Elders), and then there are the people itching for more "blue collar" jobs that aren't just stages in a progression toward a bigger "dream job."
Yeah, TS4 still needs a lot of content, and there will be a lot of players ticked off if, after all these years and all the money they sunk into the game, content they wanted to see return or new content they wanted put in never makes it in because the game got abandoned for an even more fragmented and slow-building sequel. So, no. Finish TS4 first, then we'll see if TS5 is more worth it than Paralives...