Format external hard drive to ext4 in command line [duplicate]
Solution 1:
The command is mkfs.ext4, see its man page.
If you are ok with the default settings(1), it's just
mkfs.ext4 /dev/yourpartitionhere
Footnotes:
(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.