Now, I couldn't find anything to detail how much better the S Pen is from the S4 to the S7. The S7 has a reported latency of 26ms... the S7+ ups it to just 9ms. But I can't find what the rating would be for the S4, but there's a noticeable difference in my mind as I used it.
So, I thought I'd give this little 'battle' (in the Coliseum spirit of things) and see how they fare side by side...
Then I found out you can use the pens interchangeably on either device.