I have Cf-I'm a 28 yr old mom. So I can give you the other side of the fence perspective. I think about those questions too-but the questions I'll deal with are:"why are your lungs bad, mommy...or why are you always at the hospital..."
I'm not one of those pwcf that believes on giving the 'cold hard facts'. I believe in giving the facts, the truth. But not cold and not hard. Tailored to the right age.
Simply bc children are not capable of emotionally dealing with 'facts and truth' the way adults are. I was always told the truth about my Cf as a child. 'Cf is a serious health problem, your lungs can't fight off colds and bugs the way others can so you need special people and special meds to help you'...'Thats why we do this treatment every day-so you can be healthy'. My mom also told me I didn't do anything wrong to bring it on myself-it just happened. She also always told me I could do whatever I wanted in life. If my brothers had to carry fire wood into the house in winter-I had to too.
I don't believe in telling young children that Cf will kill them, could kill them, ect. Or in 'threatening' with hospital tales. Small children are not able to process and understand that. I've seen some pwcf that were given what some people call 'the facts' at too young an age, and it can go 1 of 2 ways...the child will somehow be able to handle it (rarely I feel) and they adjust and move on.
But I think more often you see kids grow up, reach the teen 'rebel' years and they lose it. They say, well according to what I've been told, I'm going to die anyway, (wrongly assuming that) so I'm not going to do these dumb treatments...I'm going to party, do what I want...come what may. Obviously thats the opposite of what pwcf need and I've known more than a few pwcf that do irreversible damage during those hard teen years.
You've already gotten some great advice. Go with your gut when you speak to your son. Remember he's only 4! You'll help him build the facts of his life little by little.
One tactic my mom used on me that worked when I was little was the 'do it bc I say so' thing. And whether your child has cf or not, all parents do this now and then. If your lo asks you why he has to brush his teeth-you say, 'so we can keep our teeth clean'. The lo says..why...You say 'so we can be healthy'. If it persists you say-'I've told you why, and now you just have to do it'.
For a pwcf's life-doing pt, taking pills, will be part of every day life. Like brushing teeth, making the bed and so on. Its part of our day and we do it. Sometimes we as parents tend to worry and over think answers in life but little kids are pretty one dimensional in thinking sometimes.
Btw, You parents of cfers have the hardest job in the world and I respect you so much.