Decisions decisions!
I’d say stick with your DJM 400 which is much closer to a club mixer than anything MIDI, then get one X1 and a Traktor Audio 2 for external mixing. You of course need a sound card if you’re going to use Traktor in any way.
I also say stick with your current mixer until you are sure if you want to move up to a 4 channel mixer or not.
So all you need to buy right now is an X1 and a 2 channel sound card (probably an NI Audio 2).
here is my old setup as an example of an external setup mate