Apple has released an updated version of its USB-C Digital AV Multiport Adapter, adding an HDMI 2.0 port to hook your Mac or iPad Pro to an external display. Previously it had an HDMI 1.4b port.
You can connect multiple things with this adapter: An HDMI cable, a USB port for a flash drive, and a USB-C port for power.
3840×2160 at 60Hz on:
iPad Pro 11-inch, iPad Pro 12.9-inch (3rd Generation), MacBook Pro (15-inch, 2017 and later), iMac (Retina 5K, 27-inch, 2017 and later), iMac (Retina 4K, 21.5-inch, 2017 and later), and iMac Pro (2018)
1080p at 60Hz or UHD (3840 by 2160) at 30Hz on:
MacBook Pro (13-inch, 2016 and later), MacBook Pro (15-inch, 2016), iMac (non-Retina, 21.5‑inch, 2017), and Mac mini (2018)
To get these resolutions and refresh rates requires you to have macOS 10.14.6 or later, or iOS 12.4 or later. You can buy the adapter for US$69.00.