4" sounds about right. I just verified it's enough for a Garand receiver with the rear sight still on (3.8" actual diagonal measurement).
You could use something like Slip 2000 water base gun cleaning solvent. But if you are going to use petroleum solvents, then you want to get a piece of iron pipe and weld or braze a cap or a flat plate on one end. But then you still have to watch out that rust doesn't get established inside that would contaminate and take hold in a Parkerized finish. Frankly, I would also be unhappy about the amount of extra space that would need to be filled with solvent in a round pipe, anyway. I would probably go to a welding shop that could make me up a flat sided stainless container that was 2½"×4" on the inside (Garand, again) and about a yard long. They could probably make you a slip-on cover for the open end at the same time. Then you could fill it with Ed's Red for economy and leave the gun soaking for long periods if necessary.
