PQRS is a quadrilateral and \( \vec{PQ} = \vec{a} \), \( \vec{QR} = \vec{b} \), \( \vec{SP} = \vec{a} - \vec{b} \).
\( M \) is the midpoint of \( QR \), and \( X \) lies on \( \vec{SM} \) such that \( \vec{SX} = \frac{4}{5} \vec{SM} \).
If \( \vec{SM} = m(4\vec{a} - \vec{b}) \), and \( \vec{SX} = n(4\vec{a} - \vec{b}) \), then \( m + n = \):