software installation - Package missing when I run `sudo apt install ...`

I use Ubuntu 16.04. APT cannot seem to find the package m4:

root@kevin:/# sudo apt install m4

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package m4 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source


E: Package 'm4' has no installation candidate

I ran:

sudo apt-get upgrade && apt-get update

This didn't make any difference.

root@kevin:/# grep main /etc/apt/sources.list
# deb cdrom:[Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801)]/ xenial main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-updates main restricted
## extensively as that contained in the main release, although it includes
# deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
# deb-src http://security.ubuntu.com/ubuntu xenial-security main restricted