Factoring is an art. A large part of it is pattern recognition. It's like hearing the first few notes of a song you haven't heard before. You may not immediately know what group it is but if you have enough experience listening to similar music you have a pretty good idea who it is.
With enough experience factoring, hopefully you see
m4 - 9n
and think, this looks almost like a difference of squares. The complication here is the n may not be even so how can it be a difference of squares? But 9 is a square, 9 = 32 . Thus you can rewrite the problem
m4 - 9n = m4 - (32)n = m4 - 3 2n = m4 - 3 n2 = m4 - (3n)2
Can you factor it now?