I'd consider waiting. You already have two great displays (I also have two of those, and three 27"), but I think Apple hasn't released a 4k Cinema is because they're waiting for a price point on the panels, and for the standards to settle out. There's zero doubt that they will release one, and my thought is that they have good reasons for waiting, so I will too.
Yeah, I think you're right. Even though the current crop of 4K displays using the IGZO panel are nice, they're still around $3K and there must be an IPS panel coming at some point soon that will bring the price point down.