I think I got an idea how to segnificantly lessen the queue length and balance population across the 4 maps:
introduce a fifth queue called “random” or “any”
when a spot an a map opens up peek at both aplicable queues and take the player with the longer wait time
record wait time in the queue entry (if you do not already do so for statistical purposes)
if a player enqueues to random while multiple maps have open spots take him to the on with the most open spots