While they haven't directly confirm a Seasons pack would come, they have mentionned it so often I think it's pretty much a given one would come at some point. When ? How ? That we don't know, but I have no doubt something about weather and seasons will come.
For example :
https://www.reddit.com/r/thesims/comments/5409p4/do_you_think_ts4_will_get_pets_and_seasons/d7xrfca/?context=3
They wouldn't say that if they knew such a popular theme wasn't planned at all. They were fairly positive about toddlers or pets too, and we saw how that went. But how they will do it is a mystery though. There are lots of different ways they could explore the theme (like tying it with farming, natural disasters or vacation), there are different pack configurations they could use (like extra packs with new destinations). They could release an EP with Seasons, then a GP about a tropical destination, and another GP about a snowy destination for example. It would mean several packs about weather and seasonal activities, but that's what they did for TS2 and I don't think it bothered people too much.
