If projection of a =α i + j +4 k on b =2 i +6 j +3 k is 4 units, then α is

If projection of $\displaystyle \vec{a}=\alpha \hat{i}+\hat{j}+4 \hat{k}$ on $\displaystyle \vec{b}=2 \hat{i}+6 \hat{j}+3 \hat{k}$ is $\displaystyle 4$ units, then $\displaystyle \alpha$ is