The code in question was first reported by 9to5Mac, and mentions an Apple-designed display with a 4096 x 2304 resolution. While there’s no specific mention of a device, the easiest (and likely most accurate) assumption is that the 21.5-inch iMac will be upgraded to 4K soon.
Late last year, Apple upgraded the 27-inch iMac to 5K. There was no display upgrade for the smaller iMac.
In addition to the resolution, there’s also mention of a new, more powerful Intel Iris GPU and AMD Radeon processors.
While nowhere near official, this is something to keep in mind if you find yourself in the market for a new desktop computer. It may be worth waiting to see what Apple has coming.