I have a learning disability (processing) and they didn't find it till I was in 3rd grade, when I dumped everything I'd learned. (reading, writing, math). I got pulled out of class and missed even more. I'm now 30 and going back to college, and it's raising it's ugly head again.
Don't throw away the info that the teacher is giving you. It's better to have a helpful and interactive teacher, then have a teacher that doesn't care or doesn't want to deal with everything. TAke it with a grain of salt, and keep an eye on him.
At 4, your son should be on set schedule (school has already started), to make everything easier for getting up in the morning. I could cut out all junk food, fast food too. The extra and processed stuff in them can help contribute to whatever is going wrong.
Your son might not be interested in school, but paper type work is done is school now-a-days. By the time your pre-schooler starts school. They are "required" to know how to spell their first and last name, can count up to 10 or higher, and know how to say his alphabet, and when ask which letter is this. He should be able to say it.
I'm not saying it's right by any means. I think kids are being pushed and pressured way to soon. Going into pre-k, is completely different than when I started. You were taught your abc's and 123's in pre-k, not to already know them.
Like some of the other mom's have suggested, I would try to get the local project child, or whatever it's called in your area involved to just check. They would also be able to tell you if he's just not interested yet. Every child hits a point where they want to go to school to learn, but it's all different.
But if he does have the starting of some kind of learning disability, it's better to catch it earlier, rather than later. My daughter is also starting to exhibit sign of having a processing disorder. They didn't want to do anything for her till the 3rd grade, and I pushed. I'm glad I did, because she ended up being VERY behind. Even though my husband and I worked with her at home.
Good luck