Trying to optimize my screen resolution to watch 4K videos on YouTube. (monitor, screens)

[Cloudy Dayz]

First my Mac Pro wants to scale the resolution on my 4K monitor from 3840 x 2160 to 3360 x 1890, which just sucks. For the last several years I have been using a hack to force the resolution to 3840 x 2160 which seemed to work well. The 4K videos and everything else looked a lot better.

But recently I noticed that 4K videos were still not showing in true 4K. The YouTube stats show the Viewport Frames as 3492 x 1964. When I increase my monitor resolution to 4096 x 2304, the YouTube stats show Viewport Frames 3725 x 2095. Still below 4K resolution. To get the YouTube Viewport Frames to the optimal resolution I have to increase the monitor resolution to 4608 x 2596. Then the YouTube stats show 4188 x 2356. Which is above the optimal resolution.

I wish I could set the monitor so I could get the exact optimal resolution for YouTube, but that doesn't seem possible. I'm not even sure why my monitor resolution and the YouTube viewport frames are not the same, or why my monitor needs to be set so much higher than the corresponding YouTube resolution.

Monitor resolution set at 3840 x 2160, seems to be the overall sweetspot, but I also want to get the maximum resolution for YouTube videos. So I'm torn between which resolution to use. I guess I have to use 4608 x 2596 to get full 4K resolution on YouTube. That makes text a little small, but I can deal with that.

So my options are:
1. Monitor resolution: 3840 x 2160, YouTube Viewport Frames: 3492 x 1964 (way below optimal).
2. Monitor resolution: 4096 x 2304, YouTube Viewport Frames: 3725 x 2095 (below optimal).
3. Monitor resolution: 4608 x 2596, YouTube Viewport Frames: 4188 x 2356 (above optimal).

Any opinions on which I should use? Here are some screenshots, if it helps.

Monitor resolution: 3840 x 2160


Monitor resolution: 4096 x 2304


Monitor resolution: 4608 x 2596


Default resolution: 3360 x 1890

[Cloudy Dayz]

Also just on a side note. Even though my internet is fast. The speed is a pretty steady 370 mbps, I still drop frames on some 4K videos in 2160p60fps. Not all but some. 30fps videos play perfectly.

I'm using a GeForce GTX770 2GB GDDR5 video card flashed for Macs. I have noticed some sellers on eBay are selling higher performance cards with 3GB, 4GB, 6GB of memory, that I could use. Does anyone think that if I invest in one of those higher memory cards, it might be enough to stop the frames from dropping in 60fps videos?