I have 2015 MacBook and I keep reading that it supports 4k external monitors. But when I connect mine, the 4k resolution option says (low resolution) next to it and the image is fuzzy. But 1080p says HiDPI and its sharp as a tack. So why is 4k or 2k resolutions labeled (low resolution) when Apple support pages say it supports 4k? Am I missing a step?