children wearing glasses, my pet subject 
. both mine are longsighted and also have squints.
dd is 4 and has had hers since 18mths, prescription is right - 5, left - 6. her glasses correct both her sight and the squint. as she is our first, i too, was gutted when i found out. cried all the way to the opticians.
ds is 1 and has had his since 5mths, but will not wear them. he's too young to understand yet. his prescription is right - 1.25, left - 2.75 so his eyesight is not really that bad but as he's squinting in the left eye, that is what the glasses correct. i'll be happy if i can get his to wear them properly between 18mths and 2yrs.
as your ds is 4, i'm sure he'll be fine with them. there are lots of trendy frames to choose from. let him pick his own if you can, and if he ever says he hates his glasses and complains he doesn't like them anymore, rather than argue with him, go back to the optician and say he's lost them. they will be replaced for free and he can choose something different, its a good thing to do this anyway and have a spare pair. they did used to give kids 2 pairs but in most areas now only give 1.
he'll look really cute in them i'm sure, here's a pic of dd in hers
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