I have a 5 month old MB Pro 13" Retina connected to a Dell 24" 4K monitor, one of the approved monitors from Apple. It works but only at 30hz and not 60. I am running the 10.10 beta with all the latest updates but no luck. I read somewhere that someone used 2 cables and ran each half of the screen at 60hz but not what I am looking for. Their 4K scaling looks great but the 30hz is killing me.