Lacy,
Hitting the dog or tying the chicken around its neck is just going to confuse the dog.  You need to take responsibility for the dogs actions.  They dont know any better, but you should.  
A four month old Pyr can not be left alone with access to the chickens.  The Pyrs will not mature and become flock protectors until they are one and a half years old.  Until then, you need to supervise ALL interaction between the dogs and any fragile livestock.  These dogs grow big, fast, and can easily kill a chicken in play or other behavior.  You need to get them under control fast, before they are too big to handle.
While the dog is young, you need to continuously train the dog.  They need to learn the basic command (no, leave it, sit, down, stay).  They need to respect you, hence you need to respect them.  They need to learn to be able to be around the chickens and at the same time ignore them. 
The only way to do this is have them on a leash (rope, cable, whatever), with you holding the leash as you guys walk around the chicken yard.  Any, ANY, attempts to chase or attack a chicken needs to be met with a low (tone), growling of NO from you.  They need to know you mean business; hence you need to act like you are a dog growling, maybe barking the command not at them.  If that doesnt get its attention, give a correction with the leash.  The leash correction needs be fast like cracking a whip to simulate a snap from another dog.  It does not need to be hard; the dog should still be standing after the correction.
If you work with them constantly, maybe once every other day for this first year and half, you might just end up with awesome farm protection dog.  
Now, dispose of those deceased chickens, pet your dog, google some LGD sites for some more training info and let the training begin.  
Since, I have trained a handfull of dogs to do this, feel free to email me with any questions.
Jim
OH, heres my LGD, Fluffy.  She only killed one chicken by accident when she was a pup, but now the chickens can roam free without any predators issues (as long as Fluffy is on duty).