So sorry about your sick pullet. Could she have fallen or sustained a head injury? I would probably bump the heat back up to 75-80, since she may not be able to maintain her body temperature due to this illness. How much over 4 weeks is she exactly? Mareks would be very rare this age especially in a brooder, but it is possible after the age of 3 weeks. It usually is seen in chickens a few weeks older up to 6 months or older. I would make sure that she is getting enough water, and some electrolytes with vitamins or a couple of drops of Poultry NutriDrench or Poultry Cell daily would be good. I hope that she gets better. If you should lose her, I would contact your state poultry lab here from this list to get a neceopsy and testing:
https://www.metzerfarms.com/PoultryLabs.cfm