As for remembering when he is older it would depend how traumatic the episode was ,you said 'bitten ; so did it hurt him, break the skin, what. it may be time to play it down aswell so you are not putting it back into his mind soonest forgotten as far as your son goes.. the more is made of it the more he is likely to remember.
definatly speak to the 8year olds mom as he may go on and do this to other children,, he was obviously abused aswell you should tell the Mom what her son said about the Uncle , it may be that someone has to make a report ..If your son was very upset all you can do is talk to him and reassure him, he is only 4 year old, may be supervise when he is at others houses .Do make sure that it is known that the Uncle is a paedophile to protect others.