The Schroth Rallye 4-point is fine for autocross, all you need. Not SCCA legal for sanctioned track events, just know that going in. Most club track days are ok with it because the "Y" joint (really an "X") is not sewn, what they look for is metal on metal buckle.
Btw my other 6-point worked very well with stock 350Z seats, no need for special slots, and nfw you would submarine with that. I miss the 6-point in my B13 for track days but just haven't gotten to putting a bar I can attach it to safely yet, so the Schroth is an excellent compromise for club track days, and very good for autocross. You'll never settle for a stock belt again, no way.
Wish I had 6-point yesterday, something to
really plant my butt and support my thighs in the seat on the banking, there's a lot of force trying to move you toward the door at 120+ mph.
YouTube - CART PoconoNorth TT SER