I teach early childhood special education and have a student who is pretty good at identifying numbers, but is not identifying letters or showing much interest in emergent reading. He likes books and looking at the pictures or singing the songs, but not identifying letters or sounds.
The child is 4.5 years old. His speech is good — he has received SLP therapy and his receptive and expressive language are both high. Could any of this difficulty in expressing letter knowledge, but not number knowledge, be an early sign of dyslexia? Is it perhaps just a matter of being 4 and not wanting to answer the questions? Also, his parent had a learning disability as a child — could this be genetic?
The problems you describe could be the initial clues that this child might have learning disabilities. The fact that he needed language therapy adds to the possibility of this diagnosis. If this child is in your class, it suggests that he is already seen as in need of special education services. The current approach would be to try remedial interventions and see how he responds. If he does not respond, then formal studies might be done.