I think it's perfectly reasonable for a four-year-old to be able to understand "this person I care about is excited about this thing so I am happy for them even if it's not my thing!" and it's reasonably-age appropriate to model that with them too.

It honestly depends on how much of this is hyperbole.

If we're taking the letter literally, I think it's *not* reasonable to expect anybody to actually jump up and down in excitement over your good news, nor to be "very upset" when they fail to perform to expectations. (Even if all you really want is "Great news, Dad!" I think being "very" upset is too much. Mildly irked, maybe.)

But if LW is just exaggerating for effect then... I don't know.