Finding a vector component from a scalar resolute

Consider two vectors, $\tvec{a}$ and $\tvec{b}$, where $\tvec{a} = -2\tvec{i} + 3\tvec{j} + m\tvec{k}$, $m \in \R^+$ and $\tvec{b}$ is an arbitrary non-zero vector.

The angle between $\tvec{a}$ and $\tvec{b}$ is $\frac{\pi}{3}$ and the scalar resolute of $\tvec{a}$ in the direction of $\tvec{b}$ is 4 units.

The value of $m$ is