How do you format an external hard drive to ext4 via the command line in ubuntu?
The command is
mkfs.ext4, see its man page.
If you are ok with the default settings(1), it's just
(1) from the man page:
The defaults of the parameters for the newly created filesystem, if not overridden by the options listed below, are controlled by the /etc/mke2fs.conf configuration file. See the mke2fs.conf(5) manual page for more details.