Thanks! Replacing PARTUUID in /boot/firmware/cmdline.txt solved the issue.
Btw why is MBR preferred over GPT in Dietpi?
EDIT: as far as I understand from here:
the reason is compatibility with other operating systems. But still I’m not sure about this:
For 4 TiB, you need to partition the drive with a GPT partition table, which is also the default in
dietpi-drive_manager.
As I tried using dietpi-drive_manager, it simply didn’t resize. So, if I install dietpi again in future, I’ll be probably converting it to GPT manually, breaking it again, and fixing it through the method you mentioned above.
Wouldn’t it better to put an option for it in dietpi-drive_manager, or maybe automatically convert it to GPT if you try to resize a disk over 2TB?