I need help with hdr.

4K means that the display or TV has 2160 x 3840. UHD means 4k or higher. There are 5k and 8k displays which are also described as UHD but TVs are all 4k so for TVs the terms mean the same thing..
HDR is a completely separate thing. It is requires the TV to be able to produce a wider range brightness and color. That means blacker black and brighter white. It means a wider color gamut.
There are different schemes of doing this (HDR, HDR10, Dolby Vision, etc). The content has to contain metadata that tells the display how to handle the High Dynamic Range content. Consumer TVs don't even come close to the full standard so the TV has to be able to interpret the metadata and use it the best it can.
Standard HDR is static in that the settings are the same for the entire movie. More advanced HDR like Dolby Vision can be set for each scene producing a more accurate result.
There aren't many HDR monitors and they ain't cheap since they are for professional video editing.
If the display is HDR is will be described as UHD HDR or 4K HDR. Then you would want to know if it supports Dolby Vision and how well it handled HDR content. The more you spend the closer to correct it will be.