Purchase

@Lovly_irony  There are a number of approved sellers of EA games, and any that offer Sims 4 should also include a code that you can register in Origin to add the pack to your account.  You would download the content through Origin, and it would of course be compatible with your base game.  Here's the list of authorized vendors:

https://help.ea.com/en-us/help/faq/where-to-buy-ea-games-v2/

If you buy from an unauthorized third party, however, you run the risk of getting a bad code, i.e. one that was originally stolen, and having it not work.  Sometimes the codes are initially accepted but cancelled weeks or months later.  I don't know where you're looking to shop, but I would guess if you're seeing a code for cheaper than the current Origin price (I think all TS4 content is 40-50% off), it's likely too good to be true.