Apple's big fall keynote happened more than a month ago, so it's a bit late for Tim Cook to be saying "one more thing." Sure enough, though, the company just unveiled a refreshed line of iMacs, including a 21.5-inch model with an optional 4K (4,096 x 2,304) screen. That last bit shouldn't come as a surprise: The internet has already been abuzz with rumors that the smaller iMac would finally get a Retina display option, as opposed to just plain old 1080p. All told, the 4K iMac has 4.5 times the resolution of Full HD, with the same pixel density as the 5K version. Speaking of the sort, the 27-inch model now comes standard with a 5,120 x 2,880 panel, whereas 5K resolution was once reserved for a $2,499 flagship edition. Across the board, both the 4K and 5K iMacs bring a 25 percent wider color range, and upgrading to Apple's hybrid Fusion Drives costs about half as much as it used to ($100, down from $200 to $250). That said, it's a shame that 5,400 rpm HDDs are still the standard throughout much of the iMac lineup.