Do we have historical documentation of Arbela meaning four gods? I realize that literally that is what arba-illu means, but could it have been a bastardization of its older name which was Urbilum, from the ancient Sumerian meaning "city on the high"? That makes sense since it's a citadel. This could be similar to how the Kurds call it Hawler, which is a bastardization of Arbil.

Also, if it was a city dedicated to Ishtar, what were the other three gods?