I had cannibalistic birds that didn't start picking until they went to roost at night. I'd suggest a higher protein feed or a protein supplement like meat, fish or fishmeal for a little while.
Is the rooster still with them? That doesn't look that bad, nor does it look like molting and may be about a year early for Marans to molt.
If you only have 3 pullets and one cockerel then that is the problem. It needs to be closer to a 1:10 ratio.
They probably won't grow those feathers back...