\frac{dx_{1}}{dt} = \left(1 \cdot k_{1} \cdot k_{2} + -1 \cdot k_{1} \cdot x_{1} \cdot k_{3} \cdot t / \left(100 + t\right) / \left(1 + \exp\left(\left(k_{4} - x_{2}\right) \cdot k_{5}\right)\right) + -1 \cdot k_{1} \cdot k_{6} \cdot x_{1} + 1 \cdot k_{1} \cdot k_{14} \cdot x_{2} / \left(1 + \exp\left(\left(k_{15} - x_{4}\right) \cdot k_{16}\right)\right) + 1 \cdot k_{1} \cdot k_{18} \cdot x_{2}\right) / k_{1}\\ \frac{dx_{2}}{dt} = \left(1 \cdot k_{1} \cdot x_{1} \cdot k_{3} \cdot t / \left(100 + t\right) / \left(1 + \exp\left(\left(k_{4} - x_{2}\right) \cdot k_{5}\right)\right) + -1 \cdot k_{1} \cdot k_{14} \cdot x_{2} / \left(1 + \exp\left(\left(k_{15} - x_{4}\right) \cdot k_{16}\right)\right) + -1 \cdot k_{1} \cdot k_{18} \cdot x_{2}\right) / k_{1}\\ \frac{dx_{3}}{dt} = \left(1 \cdot k_{1} \cdot k_{7} + -1 \cdot k_{1} \cdot x_{3} \cdot k_{8} \cdot x_{4}^{k_{9}} / \left(k_{10} + x_{4}^{k_{9}}\right) + -1 \cdot k_{1} \cdot k_{11} \cdot x_{3} / \left(1 + \exp\left(\left(k_{12} - x_{2}\right) \cdot k_{13}\right)\right) + -1 \cdot k_{1} \cdot k_{17} \cdot x_{3} + 1 \cdot k_{1} \cdot k_{19} \cdot x_{4}\right) / k_{1}\\ \frac{dx_{4}}{dt} = \left(1 \cdot k_{1} \cdot x_{3} \cdot k_{8} \cdot x_{4}^{k_{9}} / \left(k_{10} + x_{4}^{k_{9}}\right) + 1 \cdot k_{1} \cdot k_{11} \cdot x_{3} / \left(1 + \exp\left(\left(k_{12} - x_{2}\right) \cdot k_{13}\right)\right) + -1 \cdot k_{1} \cdot k_{19} \cdot x_{4}\right) / k_{1}