Lots of children have the tics and grow out of them and lots of children have the compulsion to do things to prevent bad things happening. Did you have to avoid stepping on the cracks in the pavement in case the bears/monsters,bogeymen got you?
Lots and lots of reassurance is great but don't expect logic to work that well. Knowing how to stay healthy isn't really linked to that sort of irrational fear.
Have a think about the impact his worries are having on his life. If they are significantly affecting his ability to do everyday things you should probably asked your GP for a referral CAMHS. If it's just an irritation and he's generally happy and achieving I would try to let it pass without comment.
Children do have worries and fears and they display them in different ways. Maybe the tics, etc are your son's way of managing his worries in a more obvious manner than other children.
He goes through phases with little "tics" which can be annoying and get worse when he is worrying about something. Most of the time we just ignore these and he stops doing it after a while, recently it is needing the toilet constantly, which he's done before, physical cause ruled out by GP and in any case if he's distracted then he can hold on for hours and has been dry overnight since he was about 2.
Anyway as well as the toilet thing the latest one I've noticed is that as he walks along he will step on one foot with the other in between each step. I thought he was just doing it the other day because we got caught in the rain and his feet were wet but he was doing it today when I picked him up from nursery and his shoes today were much sturdier and not wet at all - however not only does this make him very slow at walking, it's going to wreck his shoes before long!
I asked him why he was doing it and he said "I need to do this to stay alive" and it really worried me. I had a chat with him about whether he was worried about dying, showed him how he can feel his heartbeat to remind himself that he is alive and strong and said that the things we do to stay alive and healthy are eat food and exercise. I said if he feels worried we can go and talk to the doctor and the doctor can tell him all about how to stay healthy but that he didn't have to worry because he didn't have to do anything special to stay alive because that's what his heart does for him.
I don't know if he really is worried about dying - our cat died recently but he had sort of moved out to the neighbour's house anyway and barely came in here any more, DS didn't seem particularly upset by it and he knew it was because the cat had an accident with a car rather than him being unhealthy. It's just the general nervous tics that worry me because I don't want him to be "the weird one" at school and also I don't like the thought of him being a worrier at the tender age of 4! There is a lot going on at the moment as we have an international move coming up and DP is sporadically around/not around because of this but we've tried to explain and prepare him as best we can. I hope that he settles down as we get settled out there.