Apple released updated iMacs this morning, including a 4K 21.5-inch version. But, beware if you're looking to pick up a model with one of their Fusion Drives:
A Fusion Drive, as most readers will likely recall, combines the cheap, vast storage of a traditional hard drive with the speed and efficiency of flash storage, with OS X automatically and intelligently moving files from the hard drive to the flash storage as needed to make those files more easily accessible.In previous versions of the Fusion Drive, Apple has included 128 GB of flash storage—enough to store a lot of large apps and files that you use frequently. In the new iMacs with 1 TB Fusion Drives, however, that number has been cut to an insanely small 24 GB.
I recommend going all flash storage for just a few hundred more. You won't regret it.