more specifically...

4 bolt is how many nut/bolts are used to join the outer cv axle flange to the rear wheel hubs. 90-4/92 used only 3 nuts and bolts. from 5/92ish, they upgraded to the 4 bolt flanges

6 bolt is how many bolts hold the flywheel to the crank flange. in 5/92ish, they "downgraded" to the 7 bolt configuration... which has been the crux of so many 2nd gen woe's and insults