Dark Spot in the Middle of my Mole Biopsy Site

This is most likely the mole coming back (a recurrent nevus).  Moles often have a superficial and deep component and shaving a mole off the surface of the skin does not get both components.  When the deeper one regrows, it shows up as a dark spot in the center of the biopsy site.

In general, moles should be removed in their entirely, so shaves are not adequate for total removal.  If the pathology report says the mole that was removed from your back was benign, nothing else needs to be done for it.  If it was not, the rest of the lesion needs to be excised.  I would check with your doctor.
 

After only 4 days, the darkness in the center may be a scab, or residual mole.  Wait until it heals.  The pathology report will show if there is any mole left behind.  If there is no atypia (abnormality of the cells) you can leave it alone, but if the lesion has an appearance under the microscope suggesting it could turn into melanoma, then the remainder should be removed.  Follow up with your doctor to discuss.

A shave biopsy can be an effective and simple way of removing raised, benign lesions.  However, because it removes the lesion flush with the skin, if your mole was pigmented (ie brown), then some of the pigment may be deeper in the skin and can remain following the shave.

The first thing to do is to wait for the histology of the lesion to make sure that it was benign.  Assuming that it is benign, then you can leave that residual bit of pigment in the middle of the scar, if it doesn't bother you.  If it does bother you, then you can have the biopsy site excised, although this will leave a different type of scar.  Good luck.