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??
Have you tried holding Option and clicking "Scaled" in your Display Preferences window? This should unlock the full range of resolutions your Mac thinks is available for the monitor. Not sure what that will be over HDMI, but once your DisplayPort cable arrives you should see 3840x2160.