@alvin thanks kindly for your links, but unfortunately I don't get it to work. I've tried what has been suggested in the config guide and also found the right number for my 4K display but I don't get the right result.
I've found something out though:
IMG_7837.JPG
As you can see, the tv screen itself has a picture ratio of 16:9, the displayed part from Recalbox has a video ratio from 21:9 though. Makes sense to display the "21:9" image in the middle of the "16:9" screen - yes, but from Recalbox, the tv should get a 16:9 signal but it gets a 21:9 one (tried to change that in my Samsung menue but it always states, that the signal is 21:9 and not 16:9).
How can I force the output from Recalbox to be 16:9?
Best regards,
Dan
EDIT
I got it fixed somehow. Changing the HDMI port on my tv made it work... maybe this helps someone else as well đ