M4 by itself provides only a small, but sufficient, set of all-purpose macros. M4sugar introduces additional generic macros. Its name was coined by Lars J. Aas: “Readability And Greater Understanding Stands 4 M4sugar”.
M4sugar reserves the macro namespace ‘^_m4_’ for internal use, and the macro namespace ‘^m4_’ for M4sugar macros. You should not define your own macros into these namespaces.