I'm connecting a MacBook Pro 13-in. retina (early 2015) running El Capitan to an LG 27" ultra HD 4K monitor. (LG 27UD68 to be exact.) I'm a noob, but everything I read about support for external displays says that up to 3840x2160 resolution is possible at 30Hz on an HDMI connection. Which is fine for now. But when I try to set the resolution in the display preferences, the largest option I get is 1680x1050. What am I doing wrong? This LG monitor has a Display Port input, so I've ordered a Display Port to Mini Display Port cable, which I assume will give me higher resolutions at 60Hz refresh, but is there no way to get higher resolution at 30Hz using HDMI??