The J4125 requires a specific driver version for HDMI 2.0 (4K@60Hz). Older drivers cap at 4K@30Hz. Fix:
Expand Display adapters and right-click Intel(R) UHD Graphics 600. intel celeron j4125 graphics driver
If manual installation is preferred, the appropriate package can be found on the Intel Download Center. The J4125 requires a specific driver version for HDMI 2