Each child is an individual..and no two develop in the same way or at the same pace. However I would say (based only on my own experience as a parent of a child with autism and having 10 years working with children who have severe autism ) that there is a possibility that your ds will have LDs.
Not necessarily 'additional' as such although of course children can have learning diffs separately to or in addition to their autism. Autism by its own nature can prevent a child learning in the ordinary way.. by imitation, by social interaction and so as time goes by the gap between the autist and his peers grows. Add on language delays and it gets trickier still.
Having said that, I have worked with children who present with quite severe autism in the early years who have become verbal and shown themselves to be quite able (a few have left our special school and gone to mainstream). My own son was totally non verbal at 4 and didn't show much cognitive ability either... he's now 14 very very verbal and while he has significant learning diffs, he can read, he can write at about the level of an 8 year old and so is relatively functional. I thought having a learning disabled child was the worst thing ever, but in the end DS2 has been a happy child who is learning and capable of learning..but at HIS pace.
Does your son use PECS? It is very worth going down this route as communication is key, and the more input now, the better:)
Hang in there.. be realistic but optimistic, at 3.9 he has a lot of time ahead to learn, to grow, to develop and he may well surprise you:)