This is something I am very familiar with. After the fact, as parents, we always look back and see more than we noticed at the time.
I understand your shock, fear and disbelief.
My son began selling pot when he was about 18-19 yrs old. He is 22 and A lot has happened since then, it has been a roller coaster. I do not know how he he didn't end up in jail or dead because of it during that time. He went from smoking pot to anything he could get for a while.
Last year he crashed and went to rehab. but could only stick it out a month. When he came out he talked a lot about the past. this was when I found out that he was involved with pot as a teen. I wish I had known then.
He talked about selling it, and the dangerous situations he was in.
I wish he hadn't told me that then, because now, I think he's right back in it, and now i know how bad it can be. but he's 22, I have no control, and he will have to learn the hard way I think.
If your son is dealing, ( $4000, is a lot of money for a 17 yr old to have in the closet)its pretty serious, and he is probably smoking a lot of it also, pretty much goes hand in hand.
If a school calls and states they are worried about your son in a gang, listen! They don't always do this. They must have pretty good proof.
He is 17 and you are still responsible for him legally, you still have some control.
In my humble opinion, do what you can, to get him out of that environment. He could go right back to it, even with your best efforts, but I would give it a try while I could.