9to5Mac reports that Apple (AAPL -0.57%) may be readying a brand new iMac, complete with a 4K Retina Display. The site reports that within the latest beta version of Apple's upcoming OS X El Capitan operating system, there is a reference to a system with a 4096-by-2304 ("4K") display.

This resolution, 9to5Mac's Mark Gurman says, could make sense for a "Retina" iMac. If we compare the pixel densities of displays on the 27-inch iMac with Retina 5K (217.57 pixels per inch) display and the hypothetical 21.5-inch iMac with a Retina 4K display (218.58 pixels per inch), we see that they're virtually identical, further strengthening the case that this resolution is, in fact, intended for a Retina 4K iMac.

What might Apple's new lineup look like?
Today, Apple has three basic tiers within its 21.5-inch iMac lineup. At the very low end, starting at $1,099, the company offers an iMac with a dual-core Intel (INTC -1.79%) Core i5 processor, integrated graphics, and a 500GB hard disk drive.

Moving a step up from that is a machine with a quad-core Intel Core i5 processor, Intel Iris Pro graphics (still integrated), and a 1 terabyte hard disk drive. The best of the 21.5-inch models starts with a slightly faster Core i5 processor and a mobile-oriented discrete graphics processor.

If Apple were to introduce a Retina 4K 21.5-inch iMac, I could see that particular model replacing the "premium" iMac. For the lower-end models, I'd expect Apple to stick with current display resolutions, but upgrade the processors inside of them from Haswell to their Broadwell-based equivalents. 

It looks like AMD might win a potential Retina 4K iMac
Advanced Micro Devices (AMD 0.69%) has been capturing discrete graphics share at Apple for a while now. In the recently updated 15-inch Retina MacBook Pro, Apple switched from NVIDIA to AMD graphics. Before that, Apple chose AMD's graphics for the initial Retina 5K iMac. Apple is also using AMD graphics in the cheaper Retina 5K iMac that it recently introduced.

In the El Capitan code, there are new references to AMD's Radeon R9 M380, M390, M395, and M395X graphics chips. This suggests that for a potential new Retina 4K iMac, Apple will choose an AMD GPU. Additionally, I think there's a good chance that Apple will also update the Retina 5K iMac this fall to include Intel's new Skylake processors. It seems likely that an upgraded Retina 5K Mac will get the highest end of the updated Radeon GPUs mentioned in the El Capitan code.

The high-end iMacs aren't terribly high-volume designs, but given the market segment share loss that AMD has experienced in the discrete graphics chip market, such a win should be welcome news to AMD and its shareholders.

Why Broadwell and not Skylake in the 21.5-inch iMac?
Notice that I said that I expect a refreshed mid-tier iMac to feature Intel's Broadwell processor rather than Skylake. I think this will be the case because there are some leaked Intel product roadmaps floating around that suggest that Intel won't roll-out quad-core Skylake processors with Iris Pro graphics (the high-end integrated graphics that Apple prefers) until the first quarter of 2016.

In that case, I could see Apple using Broadwell processors in the low-end as well as the mid-tier iMacs (since appropriate Skylake parts might not be available), and then using Skylake paired with stand-alone graphics processors in the high-end 4K iMacs. Under these assumptions, I'd expect the standard iMacs to get an upgrade at some point in the first quarter of 2016.