R4 compressors were used throughout 1995 model year on these trucks. The switch from R12 to R134a refrigerant was 1994. The type of refrigerant does not dictate which style of compressor was used. The "R4" name indicates it's a 4 piston compressor with the pistons arranged in a radial pattern, vs. the older A6 compressor which is an axial 6 piston design.
The R4 is *somewhat* known for leaking around that seam, once it gets a whole lot of years of usage. They usually grenade before they leak. On the other hand, the HT6 compressors used from 1996-up are more known for leaking in the middle where the two case halves meet, earning it the name "belly leaker."
Richard