what is the 4th gas line on the sending unit for?

I am installing a 93 LT1 in an older car (65 olds) and using a 95 caprice tank from an LT1 caprice..caprice sending unit has 3 lines, supply, return, and vent. What is the 4th line on the F body sending unit used for? I want to hook up the 93 EVAP system and keep it functional, do I need the 4th line? I would have

vacuum line from the engine to the purge line on the canister
vent line from the tank to the tank input on the canister
fresh air vent on the canister hanging loose
and the supply and return lines from the tank to the engine.

just what is the 4th line for? I couldnt see it on the shbox drawings.

figured it out, 4th line is for the one way check valve to let fresh air into the tank, caprice has a large 3/4" line tied to the filler neck for a vent so it doesnt need the 4th line.