My 4yo nephew talks about killing ppl and said he had a dream he killed his cousin because he wouldn't play is this normal is he capable of doing?

Normal? no. Likely? : Truthfully no, it's not likely, but it's clear that he's upset. If he's a normally well-adjusted boy he may be just a little frustrated, and probably doesn't really understand the ramifications of what he's talking about. This you can observe. However, if he shows signs of cruelty, lack of empathy, or oppositional behavior, a play therapist would be a good idea.