#### Answer

$m^4+4m^3n+6m^2n^2+4mn^3+n^4$

#### Work Step by Step

$(x+y)^n=\binom{n}{0}x^ny^0+\binom{n}{1}x^{n-1}y^1+...+\binom{n}{a}x^{n-a}y^a+..\binom{n}{n}x^{0}y^n$
Here: $n=4$,
$(m+n)^4=\binom{4}{0}m^4n^0+\binom{4}{1}m^{4-1}n^1+\binom{4}{2}m^{4-2}n^2+\binom{4}{3}m^{4-3}n^3+\binom{4}{4}m^{4-4}n^4=$
$m^4+4m^3n+6m^2n^2+4mn^3+n^4$