Change font size?

So the game itself hasn't been configured for 4k resolutions in mind as at the time of release (2014) it was still quite a niche area, generally we have focused on 1080p and although so while there are no fixed resolution settings there are a few things you can try for your current setup.

F Key
Rather than using the in game settings option for full screening sometimes the Renpy shortcut can have better results with scaling resolutions.

Change Display Scaling (Windows)
Under display in the control panel is an option called either 'change the size of all items' or 'make text and other items larger or smaller', try setting this to 150(?)% or the max setting. You will then need to restart your computer at which point you can then relaunch the game. Too big? Knock it down to 125% or the mid level and try again.

Disable Display Scaling on High DPI (Windows)
If you right click on the properties of the game's exe and under 'compatibility' there is an option for this - tick it and relaunch.

OpenGL Windowed Mode
You can also try changing the render to OpenGL by holding shift while launching the game or shift + g while the game is launched, the game will then need to be restarted but after doing so if you set the game to windowed mode you can then drag the size of the window to your ideal size.