Comment # 2 on bug 1188968 from Arvin Schnell

ext4 is not reported by lsmod since it is not compiled as a module
but instead directly included in the kernel. btrfs on the other
hand is compiled as a module.

You can check /proc/config.gz:

# zgrep -e 'CONFIG_\(EXT4\|BTRFS\)_FS=' /proc/config.gz
CONFIG_EXT4_FS=y
CONFIG_BTRFS_FS=m