Merecks

Chances are if it's Merek's, they are all infected but I would seperate any showing symptoms just in case.Here's a link to give you some more info http://www.msstate.edu/dept/poultry/disviral.htm#mar
Good luck!

If it is Marek's, you may want to considered only getting vaccinated chicks in the future.

Hmm might be a genetic disorder or they may just have a weaker immune system for some reason. I would cull the one that's near dead to end it's suffering. Keep your chin up

Mareks genetic make up is in every chick from the egg
so if they have some stressful happening
Too cold, Too hot, or just moving them they get mareks

yes each affected chick will sluff off dander after they are sick with it

about the only thing if you choose to keep them alive is to feedthem a

natural probiotic wet Mash
in the wet mash put
1/2 Vit B complex pill for total chicks
1-400 mg of Vit E capsule with end cut off and vit e squeesed into the wet mash ( these help the nervous system and can't hurt them)
for six chicks make
18 tsp of ry crumbles
18 tsp of milk any kind
1-1/2 tbsp of plain unflavored yoguart

figure what ever amt feeds each chick three tsp per chick per time feeding wet mash

do  this for a week of days
then 5 times a week 2nd week
third week
3 times a week
and fouth week and there after once a week

at same time in their water
2 qts of water
1 tbsp of apple cider vinegare
and do this for thr full three weeks
it will help them become more settledt

I've never had to deal with an outbreak but, I think this is what I would do if your going to try to save them, do the outside chores first. Then do these chicks. I think I would even try having an oversized shirt or coat to slip on before handling them plus separate shoes or pvc boots that you can wash right outside their area with soap and water. And then wash with soap and water.

what does there poop look like, I dont think it is mareks at 4 weeks do you have any other symptoms you can give?