What are the best mod websites?

@mandyply Curseforge has a partnership with Maxis for Sims 4 and is a site I would trust for this for creators I didn't previous know. I'll happily vouch for the memorial site for the late Scumbumbo's mods. You can safely download from many long-established creators' own sites, which are mostly their own Patreon or Tumblr.

Keep in mind that viruses aren't the only issues with mods. An outdated mod can cause problems with a save, and occasionally those can be permanent. If you're going to use mods, here's some advice:

1. Back up your Saves before you install new mods and before a game update. Do this by copy-pasting the folder from Documents > Electronic Arts > The Sims 4 to another place on your computer.
2. Every time you change what's in your Mods folder, delete the file called localthumbcache from Documents > Electronic Arts > The Sims 4
3. If you use more than a handful of mods, consider organizing your Mods folder with a folder per creator. This makes it easier to find mods to delete and update and helps you avoid duplicates.
4. Don't download other people's mod collections or merges. This is a good way to end up with duplicates and outdated/broken mods, and merged files make it hard to update mods or delete outdated ones.
5. Read creators' instructions. They may list other mods theirs isn't compatible with, what packs a mod requires (having mods for packs you don't own can cause problems), and how to install their particular mod correctly.
6. Read any comments on the mod for issues reported after the mod was last updated.
7. Pay attention to mod news. You can do this by following creators and following mod news lists like the one I post here.

My personal added advice, as someone who has multiple times realized I had way more mods than I use, is to add mods when you know you're going to use them, not only when you happen across them and kind of like them.