Question 292579
Use a substitution.
Let {{{P=m^2}}} and {{{R=n^2}}}
Then
{{{m^4-n^4=P^2-R^2}}}
Now a diffrence of two squares. 
{{{P^2-R^2=(P+R)(P-R)}}}
{{{m^4-n^4=(P+R)(P-R)}}}
{{{m^4-n^4=(m^2+n^2)(m^2-n^2)}}}
Another difference of two squares term.
{{{m^4-n^4=(m^2+n^2)(m+n)(m-n)}}}