I do not know any specific support for editing m4. However, if you have time to save (!) And BNF for m4, you can use the Xtext ( http://wiki.eclipse.org/Xtext ) to create a coloring and completion syntax editor with an outline representation.
Updated . The answer below shows that m4 does not have BNF or is not BNeFfable, and, of course, a short literature search does not show allegations of the presence of m4 BNF - and appears, although it is not explicitly stated that the language m4 is inexpressible in this way.
Obviously, this denies my suggestion, so I have to cancel it, unless, of course, the Xtext has not been extended to deal with such languages. Consult the Xtext website above for frequently asked questions and mailing list links.
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