Text in GUI is rendered at lower resolutions, then scaled up to your set size rather than being rendered at the correct size.
Wherever any GUI elements are being scaled, for example books, or all parts of the GUI if you have scaling enabled in settings.cfg, text will be rendered at what the game considers to be normal size, then scaled up to match your settings, meaning it can't take advantage of the higher resolution to provide better clarity and sharper text. Using Morrowind's original bitmap font, this would not make any difference, but when you use a TrueType/OpenType font, you'd expect to be able to take full advantage of the added resolution. This also applies to the console, which is rendered using a TrueType font by default. The following screenshots were taken at 4K resolution with scaling factor 2.0 set.
#2 Updated by Isak L almost 2 years ago
Tried that already, did not work, just forgot to mention it. If you just increase the size, all the ingame text will become physically larger, therefore you have to also decrease the resolution value. The screenshots at the top were taken with a size of 24 and a resolution of 50. I've attached a screenshot taken with a size of 48 and a resolution of 25. If you do it the other way around, decrease size but increase resolution, exactly the same thing happens. Unfortunately there's no way to increase just size or resolution while keeping the text the same physical size.
#3 Updated by scrawl . almost 2 years ago
I could think of two possible fixes for this, both of them need minor changes to MyGUI though.
1. Explicitely set a font size for all the text skins in the game. This is currently not working because the FontName property interferes with FontHeight in a way that I would call a bug. https://github.com/MyGUI/mygui/issues/113
2. Change ResourceTrueTypeFont to allow user adjustment of getDefaultHeight() independent of the texture size. In theory this could be added within OpenMW code (as a sub-class of ResourceTrueTypeFont that is registered with the MyGUI factory), it's slightly annoying to do that though because including MyGUI_ResourceTrueTypeFont.h adds a build dependency on freetype.