I don't know. I know this was something we could do in Sims2. And I do know that with each new game save we can specify which season we want to start off with, but I see no way to adjust the seasons in each neighborhood. Perhaps, someone who knows more may come along and clarify.
only via cheats, but you have to reuse the cheat every time the season changes or starts to change
since in sims 4 they actually made the seasons in each world fit that world, they designed it so that all worlds would get all seasons. in sims 2-3, it always snowed in winter, no matter where you were. so the ability to never have winter in a world made sense. in 4, winter isn't always just snow, but might be cooler weather (oasis springs) or rainy weather (selvadorada, sulani) so no need to worry about things like "...why is it snowing in a tropical world?"
On Sulani, it only gets cool/cold at night. Even in winter, you get warm days. It rains more, but if I can't deal with the rain I just stop it with the weather cheats or have my merman change it.
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since in sims 4 they actually made the seasons in each world fit that world, they designed it so that all worlds would get all seasons. in sims 2-3, it always snowed in winter, no matter where you were. so the ability to never have winter in a world made sense. in 4, winter isn't always just snow, but might be cooler weather (oasis springs) or rainy weather (selvadorada, sulani) so no need to worry about things like "...why is it snowing in a tropical world?"