While you have issues with "breaking your heart, and that of your 4 year old", it's important to remember who's the parent. Children take their cues from the adults around them. If you're grossed out and upset, he will be also. What did the 4 year old do before there was a grocery store with neat little cello wrapped packages? He was probably out in the yard, helping Mom pluck the chicken for dinner. If you present the processing as a matter of fact part of animal farming, it will be accepted as a matter of fact part of life. My grand dtr was around and very interested in the processing when she was only 6 years old. She'd have been part of it at the age of 4 if I'd had birds then. She was intrigued with the internal anatomy, and not at all grossed out, even though she was present for incubating the eggs, and got to be involved in raising those little fluff balls.
I agree with everything you are saying, in principal, but you are taking emotion out of it. You said it's important to remember who the parent is here and that the child will take cues from the adult and that it absolutely correct but you are not allowing for the adult being emotionally invested. I have culled when necessary and feel it is my duty as a responsible animal keeper to do this myself BUT I have sobbed uncontrollably, shaking, snot everywhere, truly grief stricken because I LOVE these creatures. I could not have portrayed the calm, controlled demeanour you are indicating, if I had tried, and would never have allowed my children to witness the procedure because you're right, you have to hold it together for them, and I just don't have the ability to do so. I'm sure you wouldn't advocate strangling the family dog and disembowling it in front of a child and to some people, like myself, this isn't vastly different (yes, different, but not vastly) xxx