\[a_{1} = 1;\ \ a_{2} = 4;\ \ \]
\[a_{n + 2} = a_{n}^{2} - a_{n + 1}\]
\[a_{3} = a_{1}^{2} - a_{2} = 1 - 4 = - 3;\ \ \ \]
\[a_{4} = a_{2}^{2} - a_{3} = 16 + 3 = 19.\]