I don't know your age, and I know that, when you are younger, having a vast network of friends is incredibly important. It was to me, at least. But four friends is nothing to scoff at, especially if they are four people you can count on and talk to and go out with. I had a pastor/counselor back in the day who said that, really, as we get older we can fully maintain only five whom we know we can count on, call up at 3am to help us in an emergency, cry on their shoulders, etc. I mean, I think I may have one or two who are like that, but he was saying five max. Point is that four friends isn't so bad

I don't think that, after two or three days of not talking to this person, it would be a bad thing to send a hello message. It would be different if you had been messaging him the last three days and he'd been ignoring you, but that doesn't seem to be the case. Maybe he just got distracted, or maybe he thought you didn't want to talk, either. So why not send a hello? After that, if he doesn't reply, I would let it be. That's just the way it goes, sometimes, and it really isn't personal.

I am more introverted as well, and I understand how you feel. You may have to be more comfortable with your own company, and not having as many people around you. That's really okay. It's more about quality over quantity when it comes to friends for us, anyway.