I checked the T480s processor the Intel-core-i58350u and the processor is supposed to be able to produce 4K resolution at 60MHz
https://ark.intel.com/content/www/us/en ... lProcessor Graphics
EDIT: I just noticed that 4K for HDMI is at 24 MHz so that explains the reason. DP gets 60MHzProcessor Graphics ‡
Intel® UHD Graphics 620
Graphics Base Frequency
300 MHz
Graphics Max Dynamic Frequency
1.10 GHz
Graphics Video Max Memory
32 GB
Graphics Output
eDP/DP/HDMI/DVI
4K Support
Yes, at 60Hz
Max Resolution (HDMI)‡
4096x2304@24Hz
Max Resolution (DP)‡
4096x2304@60Hz
Max Resolution (eDP - Integrated Flat Panel)‡
4096x2304@60Hz
DirectX* Support
12
OpenGL* Support
4.4
Intel® Quick Sync Video
Yes
Intel® Clear Video HD Technology
Yes
Intel® Clear Video Technology
Yes
# of Displays Supported ‡
3