Change ubuntu for Pi 4 from ext4 to btrfs:
-------------------------------------------
Note: you will need the btrfs-tools package which is no longer in the repositories as of 20.04, you can download the deb from 18.04

Important: ubuntu kernel only seems to support btrfs, not Raspberry Pi OS kernel. as well as Raspberry Pi OS only able to boot from PARTID's not Labels or UUID's like ubuntu!

First, un mount the partitions,
second, use the "sudo btrfs-convert /dev/sdb2" command replacing sdb2 with what the writable partitions is in lsblk.
Then in the cmdline.txt file on the boot partition, replace ext4 with btrfs,
Also, under the /etc/fstab on the root/writable partition, replace ext4 with btrfs.

You have now successfully transferred to btrfs. You can now boot into it.

----------------------------------------------------------------
sources: 
https://askubuntu.com/questions/198000/how-can-i-convert-an-ext4-partition-to-btrfs-or-other-file-systems-without-los
https://www.pragmaticlinux.com/2020/08/move-the-raspberry-pi-root-file-system-to-a-usb-drive/

Date: January 26th, 2021