It’s known that the latest version of OS X Mavericks, version 10.9.3, has provided increased compatibility between certain Macs and 4K displays. It appears as if that could, in part, be due to a change in how OS X allocates certain resources, specifically VRAM, on newer machines. As Mac4ever and MacGeneration point out, certain hardware including the MacBook Pro with Retina display from Late 2013 and the MacBook Air from 2013 and 2014 have all seen an increase in the maximum level of VRAM available from 1024MB to 1536MB!
The changes on the relevant models can be noted in the System Information application on OS X although Apple has not yet documented this increase in available VRAM in these machines in either the OS X 10.9.3 documentation or the 4K display support documentation. The change in specifications is also not yet noted on Apple’s documentation covering video and memory on each machine; the current description reflects the specifications prior to the OS X 10.9.3 software update. Specifically, the increase is from 1024MB to 1536MB, roughly half a gigabyte.
9to5Mac also compared their 2013 MacBook Air and MacBook Pro with Retina display machines against Apple’s documentation on graphics memory, for example, and have reported the increase in VRAM in OS X 10.9.3 versus what Apple currently reports as the maximum amount. The change appears to be limited to the models released in and after 2013. The increase in video resources does seem to point to the increased compatibility between the MacBook Pros with Retina display and 4K displays, 4K support does not extend to all affected machines at this point, specifically in the case of the MacBook Air, so the change could just be a result of software and hardware optimization in the latest update to the operating system.
Check out these comparisons made by MacGeneration below: