Not much, RGB performance pads, Serato Flip support and DVS support. Nothing you can’t live without unless you need those exact specs, but the S4 doesn’t have all of those features either. I think your option of finding a good used SX is solid imho.
If you were buying new, I’d say go for the jugular and buy the SX2, but in this case, the SX will do you just fine. Heck, it’s what I use currently and I feel no urgent need to upgrade. I don’t use Serato, so Flip is not an issue. I don’t do christmas so don’t need RGB lights in my pads and I don’t do vinyl anymore. Presto, my GOLD (Yikes!) SX will be with me till I get something else 😀