missmen, I'd say either is within the normal range for a 4yo (though I agree that SGB's ds sounds very bright)
the normal range is very wide, children develop at such different paces and there may be enormous differences between one child who can't read at 4 and another child who also can't read at 4 (one may have genuine problems, the other may be enormously gifted and widely educated but simply not have got to the stage where they are interested in learning)
or to put it another way, one child who can't read at 4 (but can do other things) may be as bright as another child who can
when I grew up in Sweden I had never heard of a 4yo who knew how to read- I can't imagine it was because there were no gifted children around, they were simply too busy doing other things
otoh I had never heard of a 6yo who could not bake a cake unsupervised or tell poisonous mushrooms from edible ones, either
my dcs learned to read at about the same age, relatively late for this country (but would have been early for Sweden): one of them is g&t and doing very well at secondary, the other is in the next to bottom set in primary
have to admit, for me it was not their reading that helped me gauge their relative brightness, it was the quality of their conversation: with dd you could have a very mature discussion very early, with ds you couldn't
for SGB- your ds does indeed sound bright: I'd give the teacher a little time and then be prepared to nudge if he seems like he is getting bored
but remember that Reception is very much about learning through play anyway, so time not spent reading is not necessarily wasted time: it's just time when he is meant to be learning something different