The only way that your son will get a diagnosis is if he is assessed and observed by a multi disciplinary team who have experience and expertise in diagnosing autism and aspergers. You should go to your doctor and request he refers you to the relevant professionals or clinic. Your son will need to have enough characteristics out of each section for him to get a diagnosis. Google DSM IV for Aspergers and see what the criteria is. However, as you mention you cannot understand what he is saying, that sounds more like autism as those with Aspergers have normal language development - so google DSM IV for autism as well.
I don't think the things you mention are down to behaviour because you say he is uncoordinated, and that he seems unable to follow simple verbal instructions and that you cannot understand what he is saying to you. They all indicate a problem, not behaviour, although frustration at not being able to communicate or get his needs met may lead to frustration and behavioural issues.
He needs to be assessed by a Speech and Language Therapist for his language and social interaction skills and an Occupational Therapist to assess his balance/coordination and other sensory functions.
When you see the doctor also mention that he doesn't seem interested in interacting with his peers, that you cannot understand what he is saying to you and that he cannot tolerate changes to his routine and say that you suspect he maybe on the autistic spectrum.
You need to start this process because if you cannot understand him and he is 4, he is going to have alot of difficulties when he starts school, and everyone needs to know what his strengths and weaknesses are prior to school so that supports are in place by then.
I am not a professional, I am a parent of a child on the autism spectrum.
If you have not done so already, I recommend that you share these concerns with your son's pediatrician. It is very important to intervene early, no matter what the diagnosis. It will be important for you son to get help with his speech difficulties and for you and your son's teachers to get some strategies for how to work with him to promote development in all areas.