m4 is currently handled by the shell/bash syntax highlighting which works more or less. Besides the missing keywods (which could be added in a user filetype configuration) the string highlighting does not work. I haven't found a way to get this to working with changing the configuration only.
Basically m4 does open strings with backtick quotes and closes them with simple single quotes. The scintilla lexxer does not support a different closing quote for a backtick open character. Maybe that could be added as a scintilla bash lexxer option.