Makes a lot of difference knowing you just got him and he's already four.
Being a rescue it's really hard to say what his history is.
From what you say about his manners when you first got him, I don't think he's had much socializing or any kind of training. Maybe he was attacked by other dog/s before and it makes him defensive. Could be that he feels another dog will take his place in his "forever" home, so he will protect his territory at all costs.
Not knowing his background, I think your best bet would be training classes, you'l get a tremendous amount of help and he will learn how to respect you as the Alpha---maybe he thinks he has to be the alpha and really doesn't want the job. Also the NILIF works wonders on most dogs.
just sounds like you and he need a little coaching.
Oh, there's also a special halter for dogs that act like this, you could put it on, and you would have complete control over him to break the habit.
I would prefer the classes and NILIF above all though.