It means "milestone".  Version 3.0M4 was a specific code set that formed
a well-known midway point toward releasing version 3.0.  It is a
development version and not actually released... but many people use
milestones of Eclipse anyway since they tend to be relatively stable and
Eclipse has an insanely long release cycle.

Since 3.0 has been out for a very long time, a 3.0 milestone release is
very old indeed.  The closest you could get from Eclipse's web site is
the final 3.0 release, which is slightly over a year old.  3.0M4 is
probably 3 to 9 months older than that, and is apparently no longer
available from the Eclipse web site.