Mareks disease usually only shows up between 12 and 24 weeks. Rarely it has shown up at 4 weeks of age, but that is not the norm. Here are a couple of links about Mareks:
http://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
http://partnersah.vet.cornell.edu/avian-atlas/search/disease/502
http://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
http://partnersah.vet.cornell.edu/avian-atlas/search/disease/502
I have been following it although I can't recall if I've read every reply, lol. I'm just trying to figure out what to do. I'm just a backyard chicken lover, not a breeder or anything but I still want to be able to bring in new birds or sell chicks if I have an excess from a good hatch or something like that. The chick that I lost I had purchased from a 'breeder' locally (but is a member of this site) as I really wanted one of that particular breed. It was the only one showing symptoms.