\[4 - m^{2} + 14mn - 49n^{2} =\]
\[= 4 - \left( m^{2} - 14mn + 49n^{2} \right) =\]
\[= 2^{2} - (m - 7n)^{2} =\]
\[= (2 - m + 7n)(2 + m - 7n)\]