Yes, you got that right.
m4 is used in so many different contexts, and the language itself
doesn't care, so it is really up to your application to choose a
convention. But yes, autoconf recommends ALL_CAPS, or possibly
sig_ALL_CAPS (where sig_ represents a lowercase namespace for a set of
macros provided from a single source), as the easiest to recognize.
Although the builtins default to lower case, m4 lets you rename the
builtins to whatever naming convention you'd like.
Again, that's more up to your project, and what you want to document and
maintain for your local use of m4.