I haven't spent enough time with other new four year olds, particularly those with SPD, to know if Simon's level of awareness is any more or less than others, but I do know that we have the good fortune of having an especially verbal kid. The majority of what he says comes from books, tv shows, daily events, or scripts we've taught him, he doesn't often come up with things on his own. (except while singing, but that's a whole different wonder.)
The fabulous thing for us is that he uses those scripts he's picked up to accurately talk about how he's feeling and what he needs. When he appreciated something I did for him, he told me I was a very useful mommy (Thomas the Tank Engine) and when he's exasperated, he'll say "bother," just like Winnie the Pooh.
Not only does he have the language he needs, he can tell when he needs it. We had a wonderful night on Saturday, leaving Simon in a group child care setting. When we returned after four hours, they said he did well, didn't need anything from his bag o' sensory tricks, and told them a few times that it was getting too loud. The helped him find a quiet spot, he read books, and returned to playing when he was ready. He knew before he had a meltdown, needed to scream, or run or crash, and told someone who could help. He also told me that night, as he was struggling to sleep, that "that long time made it hard for my body to sleep." I hope that as he gets older and as he learns more ways to calm down, he can be even more in tune with how the wold affects him, what he needs, and how to express it.