Newly-filled Propane Tanks empty in four days-not used much

I agree with Old Biscuit, OP most likely has a leak somewhere.

But I have a couple of buddies, both in the same CG (seasonals) that had their propane tanks "swapped"! Turned out somebody was staying in the park over the winter against owners' rules, was running out of propane and decided to help himself as long as nobody was around to catch him. Turns out somebody eventually spotted him making a "switch". Now the one guy came up with a way to cable lock his tanks, making more difficult to "swap".

Dan