Unit Outward Normal Vector of a rectangle element

Question

I know if we have a triangle with $P_1=(x_1,y_1)$, $P_2=(x_2,y_2)$ and $P_3=(x_3,y_3)$ as its vertices and edge $e_i$ goes from vertex $P_j$ to $P_k$ and $k$ stays on the left when one travels from $P_j$ to $P_k$, the unit outward normal vector on this edge defined as follows:
$n_i=\frac{1}{|e_i|}[y_k-y_j\quad x_j-x_k]$ where $|e_i|$ is the length of the edge $e_i$. I want to know that, if we have a rectangle element with these vertices: $P_1=(x_1,y_1)$, $P_2=(x_2,y_2)$, $P_3=(x_3,y_3)$ and $P_4=(x_4,y_4)$, how we can define the normal vector for this element?