A version of m4 is most likely available on any UNIX machine you happen to be working on. If you are not working on a UNIX machine, you may be out of luck (but see here).
If you use UNIX, the chances are very high that you can type "man m4" and read your version of the manual pages for m4. Because m4 is so old (in the UNIX world) it can be found everywhere and because it is not well known, it has been subject to less fiddling with over the years. Both of these are to your advantage: m4 is widespread and its syntax is very consistent across versions of UNIX.
UIUC has a copy of the GNU m4 info pages available online.
If you can't find m4 documentation for your version of m4, or if you're looking for more, look in the Yahoo! collection of on-line manual pages.
In any case, you might want to pick up a copy of the GNU m4 implementation which has some examples of m4 use as well as good documentation. Also, the GNU implementation contains the esyscmd macro which makes grabbing the output of arbitrary shell commands simpler than it is with vanilla m4. finger email@example.com to find an ftp site in your neighborhood, or look in /usr/doc/m4 if you happen to be running linux.