Did you remove the belt and spin the pulley? Are you sure the clutch plate isn't dragging on the pulley?

The bearing is replaceable, it is pressed into the pulley and staked to keep it there. The R4 is a GM / Harrison built compressor so there are bound to be tons of pics out there.

You will need a clutch plate puller / installer and snap ring pliers. I haven't done one in years but there are straight forward. Once you get the plate and pulley off , _DO NOT_remove the 4 bolts holding the bearing snout to the compressor, you will blow the freon charge.

When you put it back together, the clutch plate nut is used to draw the clutch plate onto the shaft, the nut is self locking and will never tighten up. If you do turn it until it tightens up, the clutch will be locked on.