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In the given expression

Check weather there is any common factors for all terms;

None;

Regrouping the 1^st 2 terms we have,

m² - mn= m[m - n] -------eq 1

Regrouping the last 2 terms we have,

4m – 4n = 4[m – n] -------eq 2

Combining eq 1 and 2

m² – mn + 4m – 4n = 4[m – n] + m[m - n]

= [4 + m][m-n]

Hence the factors of m² – mn + 4m – 4n are [m – n] and [4 + m]