While I don't know the d4000 specifically, (I have a MT180D) I would suggest that it be one of 2 things.
Pto control, but that would have 3 0r maybe 4 positions (low-n-med-n-hi)
or
4wd in and out (engaged/disingaged) and the location suggests this be the probable choice. fwd=engaged, aft= disengaged
This assumes your model is a 4wd, otherwise it might be for a mechanical front PTO.
Others might have more specific info regarding your specific model.
Good luck!