I've never heard of a 5th getting stolen. Just make sure your insurance is up to date. If youre real concerned, just run a case hardened chain through 2 of the wheels and install a case hardened padlock.
If they want it, no matter what you do, they'll get.
16" long 5/16" transport chain and a heavy padlock has worked for me since the '80s. I've seen some in campgrounds with a 2'-3' long chain/padlock hanging down with a red flag used when the front of the trailer has to be several feet high. The chain/red flag hanging down is a busted head saver.