The audio4 has two stereo inputs, which means you can get stuff into your computer. The audio2 only has stereo outputs, however it is cheap, portable, simple and LOUD. Although if your present soundcard is good, use that?
If you are going for external mixing, then you will need 4 mono outputs (2 stereo) at least. That gives you 2 deck control. Examples: audio2dj (2x stereo out), audio4dj (4x mono out)
If you want 4 deck control (external mixer with 4 channels) then you want a sound card with at least 8 mono (=4 stereo) outputs. Examples: audio8dj (8x mono out)
Why to get audio4dj instead of audio2dj? It has 4 mono inputs (=2 stereo inputs). These can be used to connect turntables or cd players. Even if they are not in your set up right now, you might want to consider it. Also, recording your set will be more difficult with the A2D because it has no inputs. You can route from your mixer to the audio 4DJ inputs and then record from there.
You pay, so you decide
What I would be doing is using the midi controller and the internal mixer, keeping away from the external mixer as I said before. Just to play safe with other on the decks. Also, for recording, I did know that Traktor has a recorder built into the program so I really wouldnt need to get a separate recorder for my sets. I do understand the concept if I wanted TTCD or TT.turntables, but I dont. I got into the digital concept of spinning and sold off my equipment (cdj’s). So, I want to break the barriers w the digital which I care for and its lets you play more w the music then always trying to get it to mix together.
I just dont want to spend $ on something that I wont use as much at the end. The audio 4 does sound good and sturdy but if the audio 2 does the same and cheaper, that just may be the plan!