Like you, the S4 was what got me started on IEMs.
Anything really. Those are great starter IEMs, but the rabbit hole gets deeper from there.
If you're looking at bang for the buck, you can go pretty high end with Chinese IEMs. For example, my daily driver is a 4 BA + 1 DD that I paid about $212 for, that will shame the S4 on bass, mids and treble.
If you want to stick with bigger brands, Sennheiser IE80 might be a bit outside of your price range, by $20 or so.