[Amps] what is a 4 wire 240VAC service?

[Paul Christensen]

"...All of the receptacles specified  for power
> amplifiers are 3 wire, four pole grounding.  (NEMA L14-20R)"

NEMA L14-20R is 3 pole, 4wire.

"...he said he could not endorse an 2 wire, three  pole grounding (NEMA
> L6-20R)  configuration due to liability reasons."

And L6-20R is 2 pole, 3wire.

Both locking receptacles utilize a ground conductor, so I don't know what 
liability the PE is referring to.  In any event, he did you the favor so 
that all new and most older commercial amps can be used -- although an 
L14-30R and 30A service would have been preferred if ever dragging a 77Sx 
into the shack.

Paul, W9AC