I follow the reply here to list the types of file systems that my kernel can support.
ls /lib/modules/$(uname -r)/kernel/fs
ext? My Ubuntu uses ext4 file systems.
ls /lib/modules/$(uname -r)/kernel/fs command only lists the names of the filesystem kernel modules of your currently running kernel. If your kernel has been compiled with built-in ext4 support, then there is no external kernel module necessary, so it won't show up in that ls command. It's likely your case (also because Ubuntu typically ships such kernels). For example on an Ubuntu 14.04 LTS you could do this to check:
grep CONFIG_EXT4_FS= /boot/config-$(uname -r)
This will likely result