Is the 4 for a truck and the 3 for car?
Thanks
JZ

Are you referring to rear end carriers? If so here's the difference for our cars.

GM 3-Series Car:
1965 ~ 1971
12 Bolt
8.875 Ring Gear
30 T Axle Spline
3.07 to 3.73 Gear Ratio

GM 4-Series Car:
1965 ~ 1971
12 Bolt
8.875 Ring Gear
30 T Axle Spline
3.90 and Up Gear Ratio

The difference is the Gear Ratios with each series. The ratios change a little for truck rears. See this chart here:

http://www.traction.eaton.com/prod5.htm

Just to add, that the 3-series are the "thick" gears and the 4-series are the "thin" gears. Actual difference is that the Flange, that the ring gear bolts to, is either closer or farther away from the pinion gear. (about .315") Just over 5/16 of an inch.

A 2.73 gear has a very large pinion head. Because of this the ring gear has to be moved away from the pinion centerline. A 4.88 gear has a small pinion head. Because of this the ring gear has to be moved closer to the pinion center line. Ford makes this adjustment by making the ring gear thicker or thinner. GM made the ring gears about the same thickness for all of the ratios, and used the carrier to move the gear. A 2 series carrier has the ring gear flange moved away from the pinion center line. A 4 series has the ring gear flange closer to the pinion center line.