If you plugging an HDMI cable into an Apple dongle you will NOT achieve 4K @ 60HZ. You either need a display that has USB-C or you need to purchase a Display Port->USB cable to plug directly into MBP. HDMI will only give you 30HZ. edit** If a display is HDMI 2.0 supportive.....I think this is the exception. Can anyone confirm if the display supports HDMI 2.0 then you can infact utilize a HDMI cord straight into an Apple dongle and achieve 60hz?