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