Does anyone know what the "4ST" suffix is on the acme thread designation 5/16-8-4ST? This was on a print that I need to quote. Any help would be appreciated

Have to admit that I do not know for sure but I can tell you that 5/16"-8 is not a standard thread size. I'm guessing that you are looking for a 5/16" - 8 Thread that is a Single Start and a Class 4G. I have posted an Acme Calculator that can figure the Sizes for you.

That is a four start thread. The lead or pitch is .500.

Haven't heard of that one before, but here is a chart that includes that size:

http://www.techno-isel.com/LMC/Products/AiNuts01.htm

:)