There must be a series of interrelationships among the other three factors, which restrict the quantity or index value of the factors;
Similarly, for example, a typical predator model, in fact, between food X and predator Y, food X is not an independent variable. In this model, there is a constraint relationship between them, and both of them can be regarded as functions of time parameter T. The model needs to have an initial value, and a certain balance can be achieved through the constraint relationship-differential equation.
If it is three factors, it can be regarded as a function of a parameter variable if possible. The mathematical model is constructed by differential equations, but it is more complicated than the above example, and partial derivatives may be used.
Good luck finding a good way.