I know this is an old post, but I'll offer my opinion.
I got the S4i over a year ago and liked the sound. After I sweated too much and rendered the Apple control useless, I went ahead and ordered the X10i. (Klipsch was going to warranty the 4's, but I fixed them myself and still use them with a waterproof cover over the control; I have a microscope & really small tweezers.)
When I got the X10's, I was disappointed with the bass. It was lacking as compared to the 4's. They sound cleaner and are smaller in size, but I was expecting them to have more bass.
So, that's what I think. I still like the 10's, but the 4's have more bass when I listen to my iPod.