After killing the efreeti, you take on Thorasskus, who is masquerading as a little kid. Once he's dead, there is a hole in the SE corner that opens up to a treasure room. Go down there and you'll find the fourth basin.
Nightowl2
P.S. I understand there is only a limited time to go through the door after the last basin is tainted, so you may want to do treasure-grabbing first.
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