AttributeError: 'function' object has no attribute 'rk4'

This error message is telling you that you are trying to access an attribute called `rk4` on a function object, but that function object does not have an attribute called `rk4`.

To fix this error, you need to make sure that the `rk4` attribute is defined on the correct object, which is probably an instance of a class or a module. Double-check that you are calling the `rk4` method on the correct object and that the method is defined in the correct place.