<p>Googling has turned up little to nothing.</p>

<p>I need to develop some heavy stuff in m4 and I'd love to do it in my favorite environment with all the bells and whistles thereof.</p>

<p>There are packages for running m4 on just about every platform, such as <a href="http://gnuwin32.sourceforge.net/packages/m4.htm" rel="nofollow noreferrer">windows</a>. So I know at the very least I can create a default project and test from the CLI. But I'd rather not :)</p>

I'm not aware of any specific editing support for m4. However, if you have some time to spare (!) and the BNF for m4, then you could use Xtext (
http://wiki.eclipse.org/Xtext
) to create a syntax-coloring and -completing editor, with an outline view.
Updated
an answer below suggests that m4 doesn't have a BNF, or is not BNeFfable, and certainly a short search of the literature shows no claims that an m4 BNF is available - and it appears, though not explicitly stated, that the m4 language is inexpressible in this way.
Obviously this negates my suggestion, so I must retract it, unless of course Xtext has been extended to deal with such languages. Consult the Xtext website linked above for FAQs and mailing list links.