After a few test, I think the problem come from the difference between a font size set by point or pixel (there is a lot of information on internet about this) but in a few words:
point size -> font size look same for human no matter if screen a low resolution or a 1000K resolution.
Pixel size -> the font use always the same pixel width and height no matter the screen resolution : example if screen is low resolution and font look good the same font on 4k will look very very small.
In qet we use pointSize and I think the problem come from that. Use pixelSize for diagram font is not difficult but there is some other part of qet that will be cause issue .
For the texts of elements I think I can fix it without a lot of effort, but for the independent text item this is an other pair of sleeves...
The text are automatically saved in html in the .qet file even if you don't use html formating. The html only write the pointSize of font and not the pixelSize, not impossible to do this, but probably not easy.
For fix it I need to :
-use pixelSize instead of pointSize.
-Independent text by default be a simple text and not formatted to html and when are saved, save it as simple text or html formatted according to what he was.
With these two points I think I can fix the problem of font size between hd and 4k screen.