In a 4 player game, it is usually beneficial to play to your archetype. 6 black market cards doesn't give so many options to 4 players.
For example, Katana for the street samurai, Icon Grab and Retrieval Agent for the Decker, Deathtouch for the mage and Negotiation for the Face. Ymmv of course.
Mage also has to use spells to get more spells for some reason, samurai also monowhip (the enemy you pull can't attack if you play any other weapon card that turn).
I don't think negotiate is affected by other cards. Face doesn't have any cards that I know of that benefit from focusing (maybe the expansion will fix that?), and push the advantage is slightly better if you don't. Maybe use the face to buy cheap cards to churn the black market more?
Having 4/7 cards of your type to start and 1 of the others doesn't sound like a huge difference, but even end-game the deck is probably around 15 cards. 3/15 is 20%, so it is a noticeable difference. Especially early game, and if you get those cards mentioned above.
Late game, grab whatever you can, usually an assist ability, to get the run done before someone goes down.
Edit: Wow, just re-read negotiation. Each skill card lowers cost by 1? Nice. I also saw a clarification post that says the ability also applies to purchasing negotiation, but I don't know where and it doesn't really make sense.