Can I play a fourth ally card, and then discard one of my existing allies from the table to get down to the limit? Or could I just discard an ally to make space for a new ally?

The rules don't seem to mention anything about being able to discard an ally of your choosing when or before playing a 4th. All it really states on the matter is that you can control no more than 3 at a time. Looks like you have to get rid of one of them through consequential damage, or some other means before you can play another.

EDIT: Ignore that stuff above this line, it's wrong.

It’s definitely in the rules somewhere, I can double check in a bit—you may play Ally #4, at which point you must discard an Ally unless you’ve got the Triskellion in play.

p.11 learn to play :

ALLY LIMIT

Each player may have up to three allies in play under their control at any given time.

A player who controls three allies may play a different ally , immediately discarding one of their previous allies

1 minute ago, Shindulus said:

p.11 learn to play :

ALLY LIMIT

Each player may have up to three allies in play under their control at any given time.

A player who controls three allies may play a different ally , immediately discarding one of their previous allies

Thank you! I was going through the Rules Reference instead of the Learn to Play and totally forgot about that bit.

Yeah it needs to get put in the Rules Reference, really. Also as worded you have to discard one of the previous Allies, which is fine, but I’d like clarified—I could see it being advantageous to have 3 Allies and play Mockingbird just for her Stun and then discard her. Right now it would appear that’s not an option, but I guess I’m just curious as to how intentional that is.