A particle $P$ is moving with a constant speed of $6 m / s$ in a direction $2 \hat{i}-\hat{j}-2 \hat{k}$. When $t=0, P$ is at a point whose position vector is $3 \hat{i}+4 \hat{j}-7 \hat{k}$. Find the position vector of the particle $P$ after 4 seconds.