Well usually as games go, assets usually have their own format created for that particular game or game engine. And those assets formats are usually compressed data containing models, textures and even sometimes more stuff like their associated animations (and even more complex stuff such as scripts!).
In order to create something for The Sims 4 you would have to figure that stuff out. You would have to figure out not only how to make the assets, but how to convert them to something that the game can read.
Rarely now a days you find a game that runs with ready to edit formats like png, jpeg, .obj, etc; although I have seen a couple independent games not too long ago doing just that.
So I don't think any of us here are going to be much help. However we can defiantly give you tips on how to make the assets themselves (like modeling, animation etc.).
However you take this information I wish you good luck in modding. It's something I never had very much patients for myself, as I just rather program my own engine (like I am doing right now

).