I had a Mixtrack & upgraded to the N4.
Question 1 – N4 has it’s own sound card. The DJ I/O is not needed.
Question 2 – Yes you can control 4 virtual decks with the N4 alone. You can also control 2 virtual decks + 2 timecode (virtual) decks using VirtualDJ Pro.
Question 3 – Better build quality. The faders and knobs feel much better. The jog wheels are smoother. My Mixtrack jog wheels weren’t the smoothest. These are much better.
The N4 can also be used as a stand alone 2 channel mixer.
ps-I almost picked up a Reloop Terminal Mix 4 but couldn’t justify the nearly $200 price jump. My N4 cost me $415 brand new and it’s well worth it.