Cats Leg Spasm

Question
I have a 4 month old kitten, who is playful bright and alert. but just as he starts
to relax he gets a vibrating in all 4 limbs. he doesn't appear to be in any pain as
he is purring. the vibrating stops when he is active or in REM sleep.

Answer
These don't sound like spasms, which are painful. If he is a smallish kitten it could just be that his purr is so powerful it rattle his whole body. I have seen many kittens like that. The hair on their back legs actually vibrated from them purring.

If you are concerned that it is something more, then make an appointment with the vet or when he gets his next shots, which would be at 16 weeks, ask him about it then.