If you use a solid adblocker browser addon like ublock origin (and set it to block popups if a site seems sus) that cuts out most of the concern of stuff leaking through from browsing ad-filled sites.
Sims 4 modders are usually using websites like tumblr, patreon, simfileshare, workupload, google drive, modthesims. Or in the case of CC, maybe stuff like thesimsresource.
These are all pretty trustworthy, IME (I think most of them probably scan for viruses when the person uploads a file), and the more modern OS's like windows 10 are probably gonna jump in and try to do a virus scan on what you download anyway. If you're extra worried, you could run virus protection in the background, but it might be overkill and could result in false positives if it's one of those awful ones like norton antivirus. I've had a pretty good experience with malwarebytes, though unfortunately it doesn't allow a full scan without paying anymore.