i'm not sure who manufacture's these masts or cylinders for crown but i have seen in certain cases where they put a safety valve in the bottom of the cylinder and have seen these valves come out because of snap ring failure. This might be your problem, if the valve is sitting in the base of the cylinder it could be keeping it from lowering all the way down.

The only other thing i can think of would be something catching in the mast channels but you already said you checked for that. So that is what makes me think it is something inside the cylinder.

The best say to test to see if its one cylinder is to block and chain up the mast and take the cylinders loose at the top where they are bolted to the innerchannel and then lower them down and see if both of them get stuck at the 4inches or if it is what you suspect, that only one is hanging up.

WARNING: If you are not familiar with working on masts and do not have the proper equipment to chain and block up the mast then call a qualified service person to do this for you. Working on masts is an EXTREMELY DANGEROUS task and should ONLY BE DONE by a qualified person.