Why does windows 10 not support 4K?

Windows 10 Supports 4k..... I'm running it with a 4k monitor.

The problem is either you don't have your settings right or your Graphics Chip/Graphics card doesn't support 4k. 

right click , display settings

scale and layout

Change the size of text, apps, and other item

100% (recommended) 

If you change this it would make the text and icons bigger.

4K is just a size. What you probably meant to tell us is what Video Encoding the file has. If you install VLCplayer which plays most file types it should work fine.

Although I hate win10 that much, your issue is with hardware. Win10 supports 4K as long as the GPU can handle it meaning you should change your graphics card with a much more expensive one right away.

Your problem is NOT with Windows 10, it is with your GPU.   Change the Settings or invest in a better GPU.

It does support 4K. Stop being a moron.

"The text and image are way too small."

So make them bigger. Right-click the desktop, select "Display settings." Under "Scale and layout", change the dropdown box labeled "Change the size of text, apps, and other items" to a larger value. If you need it even larger, select "Advanced scaling settings".

"How can I lower the resolution but have more pixels?"

That's an oxymoron. Not to be confused with a moron.

"Apple does not have this problem where the text is very small."

Then buy a Mac, like all the other idiots out there who are too stupid to change a basic setting on their computer.

"When will windows 10 support 4K?" 

It does. It has for decades.

Windows 10 does support 4k. You video chip and monitor have to support it too though. Buy a better graphics card and a 4k monitor.