If $\overrightarrow {a}=\hat i+\hat j-\hat k$ and $\overrightarrow{b} =+ \lambda \hat i- 3\hat j+\hat k$ and the orthogonal projection of $\overrightarrow b$ on $\overrightarrow a $ is $\large\frac{4}{3}$$ (\hat i-\hat j-\hat k)$, then $\lambda$ is equal to