I keep hearing that the S Pen will not work on a Galaxy S4! Funny, I've been using it on my S4 since I got it a few months ago! And, before that I used it on my S3! Things that make you go hmmmmmmmm! It works, it ain't perfect, but it does work!!
There are many aftermarket stylus that will work with the S4 and other devices. The selling point behind the Note line is that it was designed to incorporate one and had features that take advantage of it.
Good to hear that you are enjoying it with your S4.