Try changing the HDTV settings to Resolution 1920 x 1080 @ 60Hz also in Windows Settings - Display. See if that makes a difference.
Try running at 3840x2160@60 Hz and see if it works better after trying 1920x1080@60 Hz. From what I have read it will work with 4K Resolutions but maybe you will need to lower the Hz to make it work correctly.
The Specs on your IGPU Graphics for your Motherboard: B450M DS3H (rev. 1.0) | Motherboard - GIGABYTE U.S.A.
Here is the specs on the RX Vega 11 Graphics:AMD Radeon RX Vega 11 Specs | TechPowerUp GPU Database
EDIT: Since you didn't post your Computer specs nor HDTV Model number I am assuming you don't have a GPU card installed and are just using the IGPU of the Processor for your main Graphics Display.
If so, make sure in BIOS/UEFI you have the Maximum RAM Memory assigned to the IGPU of 2 Gbytes. This will make the IGPU run better and faster.
Possibly add more RAM Memory from 8 Gbytes to 16 Gbytes will help speed up your computer. If your IGPU is using 2 Gbytes of RAM Memory than that leaves only 6 Gbytes of RAM Memory to run Windows and all your other apps.