It depends, I would say.
It can work like here: https://www.reddit.com/r/dietpi/comments/tadys7/pi3b_pi4_upgrade/
This thread mentioned it is not possible, but this post is somewhat old.
Make a backup and just try it 
It depends, I would say.
It can work like here: https://www.reddit.com/r/dietpi/comments/tadys7/pi3b_pi4_upgrade/
This thread mentioned it is not possible, but this post is somewhat old.
Make a backup and just try it