Thank you very much for information. Yes i do quarantine my new birds,these little ones are in quarantine now.If you can order things in from the States, you can order it from Jeffers. I'll post the link at the end. The MD-Vac, the only one we can get for home use, is Turkey Marek's live virus. Chickens cannot get Turkey Marek's so the vaccine works the same as the Cow Pox vaccine used on some of us to keep us from getting Small Pox. Humans can't get Cow Pox but it is close enough to Small Pox to provide us with immunities. MD-Vac is not going to give your birds Chicken Marek's, it can't, it's just not in there.
At 4 weeks of age, any resistance from parent stock is pretty much worn off and if the parents didn't have it and survived, your chicks are at risk as is anyones not vaccinated. I can't say your chicks have been exposed for sure, but I would bet they have been. So knowing this, I would say that yes, if you can get it, it might be worth the shot, no pun intended. Another thing you can do is get a turkey or two and run them with your chickens. My experience proves to me that this works, probably better than the vaccine would for you now. Everyday, your chickens are going to be 'revaccinated' by being with the turkeys. I do highly recommend that you get the meds you would need for the turkey just in case Black Head shows up in it. Chickens can have it but it is rare and doesn't get them as badly as it does turkeys. I have had 2 bottles of Fish Zole [metronidazole], also from Jeffers, sitting here for almost a year now, "just in case".
Now the disclaimer...you knew there would be one, even if you vaccinate, run your chickens with turkeys, practise biosecurity until your eyes cross....sometimes a strain or strains will come along and explode in your face. When this happens, there's not a darned thing you can do about it except pray. This is what happened to me here. Don't get the wrong idea, I am not trying to discourage you, far from it! I just want you to know that even if everything is done properly, vaccinated at birth, kept away from other birds for the 6-8 weeks needed for the resistance to build up, good biosecurity, it can still go wrong. These mutated strains don't go by the rules and have a way of compromising the defences already there with the vaccinations. So back to your question, should you vaccinate now if you can get the vaccine? Why not? You have nothing to lose and everything to gain.
http://www.jefferspet.com/md-vac-cfl/camid/LIV/cp/F7-J7/cn/3501/
One last thing....the blank stares were your first clue. In the future, please make sure you know the history of the breeder and their birds. Easier said than done? Not really, ask others involved with poultry and showing their birds. Look at the parent stock and the facilities if you can. Go over the bird with a fine tooth comb. Finally, if you do purchase birds, quarantine those birds for 6-8 weeks. Completely away from the rest of your birds and change or clean footwear and clothing before going into your other birds. No it's not easy, it's a big pain you know where, but it may save the lives of your other birds. Sorry, lol, I do go on.