[Bigdav160]

The R4 compressor is completely fine for use with R134a. GM used it that way in '94 and '95 models.

You'll want to change the drier to a new one. The old desiccant in it is not compatible with R134a.

There's a chance the TXV (expansion valve) is stuck from sitting with no lubrication. The POA valve will need to a pressure adjustment to work effectively with R134a.

Likely, you'll want to change the hoses to the newer style (takes a little tweaking to fit with the R4 compressor) and while your at it, update the condenser to a more modern design.