A quick post for others who may be look at this. Yes, the Dell P2415Q does work in full Retina display mode with your late model MacBook Pro with Retina graphics.
Ever since I got my MacBook Pro with Retina last summer, I’ve been anxious awaiting when 4K monitors reached the quality/price nexus for purchasing. Graphics on the MBP are great, but I want the big desktop size, and I am very used to dual screens.
With the recent release at the end of 2014 of the Dell P2415Q, I was ready to go, especially after a January price drop brought it to $529. I pulled the trigger, but couldn’t figure out how to get it in Retina mode. I found some hackintosh articles and an Apple support doc pointing to the more expensive UP2415Q as being supported, but not the specific answer.
After a few detours, I figured out that I was using the wrong cable! Reminder: for 4K resolution, you must use the HDMI ports, not the mini display ports. This solved my issues right away.
On the plus side, while reviewing the specs on the P2415Q at dell.com, I found the price had dropped to $449. I am working on getting my price guarantee refunded now.