Reply to Traktor S4 or 4-channel mixer?

Howdy, I'm seeking some advice for upgrading my current setup:

Audio 2 DJ (going to my home theater amp and speakers)
VCI-100 SE (with default DJTT mapping)
Traktor Pro, internal mixing mode

Two reasons I want to upgrade:

1. I am buying monitors, so I want a better way to connect them (direct cables from the Audio2DJ would work fine, but I'd also like to be able to connect other sources to the monitors on occasion)

2. I want to start using the 3rd and 4th decks, mainly to play loops, but I find it too hard to do with internal mixing and the VCI-100

Traktor S4 would cover all the bases. I could sell all my stuff (I suppose I could get a few hundred?) and just have an S4. My only concern is that it's way bigger than the VCI-100, which I do occasionally take in my bag to parties etc.

Alternatively, I could get an inexpensive 4-channel mixer (I see on craigslist a Behringer DDM4000 for under $300) and 4-channel audio interface. Then I can keep the VCI-100 and use external mixing.

What would you recommend? Especially those of you who have gone from a VCI-100 setup to an S4?

I haven't generally messed around with the VCI-100 midi mappings, I'm not doing anything fancy enough to need it. I also don't do a lot of controllerism/cue juggling. I am mostly just mixing, using fx, and want to mix in more loops via deck 3 and 4.

TIA