Actually, what you can download are "saves" which are re-created worlds. Lana's CC finds has the best list of them, but one is missing. If you want the one that is missing, PM me, but all the other links are intact. The one that is Pleasantview 15 years later is absolutely super fun to play, but there are more:
http://sssvitlans.tumblr.com/tagged/s4worlds
Jenba's are fun to play as well, and she just uploaded Maristella.
The way it works is that someone will rebuild/rename the worlds, play them for a while to kinda build a story, and then put them up for download. Some are just rebuilt (like mine, but my skills are basic). The key is that you want a
backup of YOUR saves, because you don't want to overwrite your save with a download that has the same number. For the most part, those who have offered are pretty savvy about re-numbering the save file so that it shouldn't overwrite yours, but you should be aware.
When someone offers the download, you simply unzip and put the save file (without the unzipped folder) into your Saves document folder.
If you need to rename, make sure you have the same AMOUNT of numbers in the save file or you will make it unusable.
For example: This is what a save file looks like Slot_00000002.save You can rename it Slot_0000456a.save or Slot_48000001.save