\frac{dx_{1}}{dt} = \left(1 \cdot k_{21} \cdot k_{1} \cdot x_{1} + -1 \cdot k_{21} \cdot k_{1} \cdot x_{1}^{2} / k_{20} + -1 \cdot k_{21} \cdot k_{3} \cdot x_{2} \cdot x_{1} / \left(k_{4} + x_{1}\right) + 1 \cdot k_{21} \cdot k_{8} \cdot x_{4} \cdot x_{1} / \left(k_{9} + x_{4}\right)\right) / k_{21}\\
\frac{dx_{2}}{dt} = \left(-1 \cdot k_{21} \cdot k_{14} \cdot x_{2} + 1 \cdot k_{21} \cdot k_{2} \cdot x_{1} / \left(1 + k_{13} \cdot x_{4}\right) + 1 \cdot k_{21} \cdot k_{6} \cdot x_{2} \cdot x_{3} / \left(k_{7} + x_{3}\right) \cdot \left(k_{6} - k_{17} \cdot x_{4} / \left(k_{18} + x_{4}\right)\right)\right) / k_{21}\\
\frac{dx_{3}}{dt} = \left(-1 \cdot k_{21} \cdot k_{5} \cdot x_{3} + 1 \cdot k_{21} \cdot k_{15} \cdot x_{2} \cdot x_{1} / \left(\left(k_{12} + x_{1}\right) \cdot \left(1 + k_{11} \cdot x_{4}\right)\right)\right) / k_{21}\\
\frac{dx_{4}}{dt} = \left(1 \cdot k_{21} \cdot k_{16} \cdot x_{1}^{2} / \left(k_{19}^{2} + x_{1}^{2}\right) + -1 \cdot k_{21} \cdot k_{10} \cdot x_{4}\right) / k_{21}