With the addition of the Leader's expansion, you can use your leaders to build your wonder. If you build Babylon (b)'s second stage (which allows you to play the card you would normally discard) with your third Age leader, can you use this ability to play your fourth leader or must it be used to play a third age card? As the fourth leader would normally be discarded, the question was raised as we were playing tonight. I'm not sure how answer it since, if you were using a leader in any other age, you would not be able to play another leader as you would not be discarding a leader at that time.
No, you are not allowed to play a Leader.
From the base game rules, page 9:
From the Leader rules, page 3:
See the bold parts: Babylon (B) allows you to play an Age card, but Leaders are not part of the Age card set.