strange --version issue

Can someone please tell me what the heck is going on here:

$ echo $PATH
...:/usr/local/bin:/usr/bin:...
$ ls -al /usr/bin/m4
-rwxr-xr-x 1 root root 60768 2006-06-16 07:39 /usr/bin/m4
$ ls -al /usr/local/bin/m4
-rwxr-xr-x 1 root root 179252 2008-04-09 08:46 /usr/local/bin/m4
$ which m4
/usr/local/bin/m4
$ /usr/bin/m4 --version
GNU M4 1.4.4
Written by Rene' Seindal.
...
$ /usr/local/bin/m4 --version
GNU M4 1.4.5
Written by Rene' Seindal.
...
$ m4 --version
GNU M4 1.4.4
Written by Rene' Seindal.
...

I'm sure this is not really an m4 question, but I'm totally stumped - 
/usr/local/bin comes first in my PATH, yet when I execute "m4 --version" 
I seem to be picking up the 1.4.4 version in my /usr/bin directory. What 
gives?

John