I'd like to second all of the above telling you not to be discouraged. An acquaintance has a kid who was non-verbal and everyone said "Look, we know you want the best for him, but he's a mentally challenged 3 year old in an 8 year old's body and he'll never be anything more as much as you may want him to be. The best thing you can do is let go of all those hopes you had and just keep him comfortable until it's time to put him in a home". She almost did, she was about to, she'd given up and was making a, well she was making memento about her son as a goodbye, because she'd accepted they had to let go of those expectations but wanted something tangible to remember it all by when she found something that worked in reaching him! Now he goes to a normal school, they ask him questions and he can indicate what he likes and doesn't like and he's even improving in communicating more verbally, and this has been just a few years happening. And he isn't a 3 year old in a 12 year old's body who will always just sit in a corner stimming constantly between screaming like those doctors said, he's a smart kid with his own views who's finally getting a chance to show that to people.
It's an extreme example and every kid won't be like that although it would be nice if every story had that happy an ending, but it shows that all those specialists and doctors - sometimes they get it wrong, really wrong. Your son is 4 years, he's still got a lot of growing to do. The assessments can be wrong or overcome with a lot of work. And with a child that young I question how accurate that test will on his development in even just a few years. Kids on the spectrum don't develop quite normally, there may be nothing for a long time, and then suddenly one massive leap forward. Don't give up, things can change a lot with some work for kids on the spectrum, even the ones all the doctors say are at the extreme low end.