How can I use 4k UHD tv as Mac mini monitor?

Assuminh yo have the 2014 Mac Mini then yes but you only get 30 Hz

Video Support

Support for up to two displays at 2560 by 1600 pixels, both at millions of colors

Thunderbolt digital video output

• Native Mini DisplayPort output
• DVI output using Mini DisplayPort to DVI Adapter (sold separately)
• VGA output using Mini DisplayPort to VGA Adapter (sold separately)
• Dual-link DVI output using Mini DisplayPort to Dual-Link DVI Adapter (sold separately)

HDMI video output

• Support for 1080p resolution at up to 60Hz
• Support for 3840-by-2160 resolution at 30Hz
• Support for 4096-by-2160 resolution at 24Hz

DVI output using HDMI to DVI Adapter (sold separately)

I'm using a 4k monitor with my mac mini 2014, however, everything is downscaled making it very difficult to read or view anything.

I ended up dropping the resolution to 1080p which is just disgraceful of Apple.

Apple needs to sort this out!