HiDPI is try to use 2x2 pixel to present 1x1 pixel

The traditional way to enable it is to use Quartz Debug. But actually you can use defaults to do so.

sudo defaults write /Library/Preferences/com.apple.windowserver DisplayResolutionEnabled -bool YES;
sudo defaults delete /Library/Preferences/com.apple.windowserver DisplayResolutionDisabled;

Then, logout and log back to take effect.

Now go to Apple menu --> System Preferences --> Displays --> Display --> Scaled. You'll see a bunch of "HiDPI" resolutions in the list to choose from.