MBR partition tables are limited to 2 TiB disk size. For 4 TiB, you need to partition the drive with a GPT partition table, which is also the default in dietpi-drive_manager.
Our RPi images however ship with MBR partition table, as RPis do not (reliably) support booting from GPT. There are ways via hybrid MBR/GPT, and the RPi 5 SPI bootloader might support GPT natively, but also with constraints regarding compatibility with how e.g. Windows reads it, thus the ability to access the FAT partition from other OSes. Overall, it causes more issues than that it helps.
So as system drive, max 2 TiB minus boot partition minus metadata is usable. Additional drives can be formatted with GPT to access their full size.