What a sad situation...I can't imagine losing a child of any age, especially while on what I'm sure was a very special family vacation and at the happiest place on earth. I couldn't imagine spending the rest of my life being reminded of what happened every time I saw any reference to Disney...my condolences to those affected by this, especially the family.
The average height of a 4 year old is 42"...I don't know that putting an age restriction on a ride would be the thing to do because every child is different. On our first visit to WDW my middle child was 5 and she was a daredevil, would ride anything if tall enough. Tower of Terror was her favorite on that visit. I don't think I would have questioned allowing her to ride M:S (if it was around then) simply because of her age, because she loved thrill rides. And still does at age 10.
My son is small for his age, and at 6 last summer he rode M:S. He wouldn't ride any other thrill ride (he didn't even want to ride Pirates a second time because of the big "drop"), yet for some reason, he loved M:S. He was not at all affected by it, even though I felt pretty ill for an hour or 2 afterward.
I can understand a height requirement on any ride. But it would be difficult to determine an age requirement because bodies and mentalities vary so much. I think once the height requirement is met it should really be up to the parents to make the decision for their child. I'm sure in this case the parents are rethinking their decision, but I can't imagine that any parent would ignore the warnings if they knew of a health condition (ie. heart problem) that could jeopardize their child's well being.
It sounds to me like this will be a situation like the death on (or shortly after) Body Wars, with an undiagnosed health condition causing the death, and not the fault of Disney, the parents or the child. As tremendously sad as that is, I'm sure nothing could have prevented it.