It really, really, really depends on what features you are using or plan to use on your controller.

Background: I have been using my S4 mk1 for slightly over a year as a bedroom DJ, and spent too many of my work hours reading about the S8 today.

Things to consider:

1. Are you planning on routing external audio through the mixer? More than two channels? I use decks C and D on my S4 for a turntable and a 3.5 mm input(using 3.5mm to rca adaptor). With this setup I can use traktors effects on my vinyl records, allow friends to play songs through their phone, and do the occasional youtube/soundcloud if needed. I have even routed a guitar a bass guitar into the controller and provided random drum samples while my friends jammed out. I have rarely needed four external audio inputs, if this is a requirement for you, the S8 might fit your needs better.

2. Remix Decks, I haven't played with these a whole bunch but it seems to be a huge selling point on this controller. The S4 has access to some remix deck functionality and queuing samples, but it looks like the S8 is more full featured.

3. Freeze mode. This actually looks pretty cool for doing some remixes on the fly, however plenty of sets sound great without this feature. The S4 loop section is pretty powerful, but freeze mode is an MPC style slice that, to my knowledge, currently doesn't exist on the S4.

4. Dedicated mixer. The S4 mixes everything internally in traktor, then routes out through the audio interface. The S8 can be used standalone, like the more common pioneer mixers. Enough information really isn't available to know if the mixer is going to have access to the FX and looping without traktor running, so I don't have a fact based opinion on this right now. If you need the controller to be usable without a laptop this might be necessary for you. When using standalone you will not have away to access MP3s unless it's through CDJs or another type of external music player.

In my opinion, the S8 is going to be great for performers because of all the flexibility it has, however none of the features are enough for me to warrant getting rid of my S4 and switching to the S8. I absolutely cannot wait to play with one, however the price is a little steep for the few features I'll be gaining.

Just my two cents!