Hello, It is difficult to understand how you can get 4 bad compressors,but
I think I have an idea.First, have the shop people put a dye into the a/c
system? Dye will tell for sure where your leak is.It could be an 0-ring
that is loose somewhere. I think uyou have about 8 or 10 of them to check.
Assuming this has happened,  your comperssor(s) may have gone past
their "shelf life" , before being installed.A compressor has a shelf life
of about 18 months before it will need to be rerepaired. The shaft seal
will shrink up and leak after this time. This is why owners manuels say to
use yur a/c  a little each nomth in winter.It keeps the shaft seal
lubricated. What I would do if in your place,is to return the compressor
and buy a new,not rebuilt compressor from a high volume business. They
cannot keep compressors on the shelf long enough to begin leaking.Good
luck, Roger.