I have to differ with the poster who says children of 4 don't think in words. At this age they are using the discourse they learn from those around them to structure their thinking so he may well be hearing his inner voice which is developing from the real voices he hears around him.
My DD1 hears voices which are a feature of her imagination. She, like Marnes DD, has AS and CAMHS have looked into it with her and concluded that it is just a peculiarity of her processing.
Both of my girls also hear voices repeated later in the day which they have heard when stressed. This is often people shouting or children screaming in the playground.
So you have three possible reasonable explanations which shouldn't concern you.
Having said that the same poster makes a good point about early intervention. If there is a more concerning explanation, the earlier he is assessed the better his outcome could be. Be warned, though, that parents seeking this sort of help are often asked to wait for months or even years to see how the child develops before they are properly assessed. This can be really frustrating if you feel your child has a serious problem.
My advice would be to see your GP without your DS and ask for a referral to CAMHS. This will probably take several months. In the meantime make notes of the occasions he mentions so you have some evidence to share with them at the first appointment.
Try not to raise the subject with him and be very nonchalant if he does share anything with you. The last things you need is for him to be frightened of the voices.
I hope this turns out to be something and nothing and you can put it all behind you very soon.