Now, as Apple gears up for its big event next week, 9to5Mac reports that the company is likely to release that 21.5in version of the iMac in October. It's slated to carry a resolution of 4096x2304, and the site's sources say it will indeed be Apple's smaller iMac that gets the 4K refresh next month.
That makes sense, of course, as the 27in iMac already pushes past that boundary with an optional 5K display. Currently, the 21.5in iMac is available with a 1920x1080 display, so embracing 4K resolution will be a dramatic upgrade - albeit one we suspect will be optional, since making 4K standard will surely kill the entry-level pricing of the lower-end models.
An October release also sounds right on the money: both last year's OS X 10.10 Yosemite and 2013's OS X 10.9 Mavericks had October public releases, and El Capitan seems certain to follow that trend. With iOS 9 sure to release in the next couple weeks alongside the iPhone 6s, it makes sense to spread the love around this autumn.
It's unlikely that we'll see the new iMac at the 9 September event, given the reported spotlight on the iPhone 6s and 6s Plus, new Apple TV, and perhaps even the long-rumoured iPad Pro. But if you've been holding out for a new Apple desktop, we strongly advise giving it a few more weeks.