A company called Wasabi Mango has released the world's first commercially available 4K monitor with a 120Hz refresh rate, beating some established players to the punch, including Acer, Asus, LG, and Samsung.
It's a large size 43-inch monitor with a simple 'UHD430' model name. It has two DisplayPort 1.4 connectors, which support 4K at 120Hz (the DisplayPort 1.4 spec also supports 8K at 60Hz), plus three HDMI 2.0 ports, a Toslink optical audio output, and a USB port.
The UHD430 uses an IPS panel and is advertised as supporting HDR, though the brightness level is only 400 nits. That's the baseline for VESA's entry-level DisplayHDR 400 certification. Note, however, that this is not DisplayHDR certified.
It also lacks G-Sync support. One reason that is notable is because Nvidia and its hardware partners are expected to release 4K 144Hz monitors with HDR and G-Sync very soon, including the Asus PG27UQ and Acer X27.
The UHD430 gets ahead of the pack, albeit with a slightly slower refresh rate compared to those two models, but also a larger display size. Still, it's probably better to wait a bit to see what comes out in the weeks ahead.
If you're really eager to grab a 4K 120Hz monitor, though, you can find this one on Ebay for $1,399.99.