Intel UHD 630 supports 4k at 60 Hz.  

3070 MT/SFF has DP1.2 (supports 4k 60Hz) + HDMI 1.4 (4K (4,096 by 2,160) at 24Hz, 4K (3,840 by 2,160) at 30Hz)

are you connecting 4k monitor to PC via DP1.2 ?

https://www.intel.com/content/www/us/en/support/products/98909/graphics/graphics-for-7th-generation-intel-processors/intel-hd-graphics-630.html

https://topics-cdn.dell.com/pdf/optiplex-3070-desktop_owners-manual6_en-us.pdf

3070 MT psu is 260W. It has a blue PCIex16 slot that can support 75W.

GTX1650 can be supported in 3070 tower.  It is a 75W card.  Recommended psu is 300W, butRecommendation is made based on PC configured with an Intel Core i7 3.2 GHz processor. Pre-built system may require less power depending on system configuration.  you have an i3 which should need less power.

1050ti is also a 75W card.  If you want to upgrade gpu for 9th gen cpu, go for the newer 1650.

I connect it via HDMI port,

I use KMplayer which crashes when I open a video file with 4K HDR HEVC.

the same videos worked fine with VLC and media player classic. but KM has better colors as VLC has pale colors and media player classic has a red tint.

I thought KMp crashed coz of the GPU but that seems wrong !!

any idea how to fix this? or a better solution for 4K HDR video playback? a better video player or a codec? (windows 10)

thank you

this is a good link to read how to enable, play back or stream HDR on Intel graphics 

refer to “activate native hdr mode on Windows”

try use movie and tv app to test play back of local hdr video file

https://www.intel.com/content/dam/support/us/en/documents/graphics/HDR_Intel_Graphics_TechWhitePaper.pdf