\frac{dx_{1}}{dt} = 0\\
\frac{dx_{2}}{dt} = k_{3} \cdot \left(x_{4} - x_{2}\right) \cdot \left(k_{4} / k_{3} + x_{2}^{2}\right) - k_{2} \cdot x_{2}\\
\frac{dx_{4}}{dt} = k_{1} - k_{2} \cdot x_{2}