Hi,
Im no expert but I manage to get 4K60fps on my RPI4 by :
- (obviously) Enabling the 4k60fps feature in config.txt : hdmi_enable_4kp60=1
- Using a HDMI 2.1 cable
- Enable HDMI UHD Color feature on my TV
The UHD Color feature was the last thing I did that finally resolve my problem.
Hope that helps.