Shear2001,
With all due respect, from the questions you are asking, you need to read all the threads on this subject that are available. It sounds as though you may not have done any A/C work in the past. (I'm trying to say this most graciously).
Even if you use a new R4, there is lots of additional information that if not followed will result in a bad installation. Then you will start a thread saying you didn't think a new R4 was worth the money because the system failed again. We ARE trying to save you some effort and hassle and money.
The basic questions are: Why did you go through 3 compressors? Leak at the front seal, a seized compressor, a pressure switch that isn't triggering the system off, leaking hoses, leaking o-rings, leaking everything else? Contaminated system......the list goes on. How do you know there are no leaks in the system? How did you determine the condition of the system?
All of this information can be found here at the site using the search engine. You can also read the posts at
www.aircondition.com for more information.
I think you need to determine first why you went through 3 compressors before you start asking about o-rings.
(I hope my writing style was not too harsh, that wasn't my intent)