[RStar]

As far as introducing a 4 year old or a kitten, I'd suggest the kitten. Grown cats seem to not be threatened by them, and the kitten hasn't yet developed an attitude, so it's not bothered by any of it. This way there's no real fighting.

After a while the grown cat will tend to parent the young one. Also, the youth of the younger cat helps keep the older cat youthful in attitude and play. As NA suggested a 4 month old is good because if the older cat plays a little too rough the younger one can handle it better.

At least this has been my experience (except mine were age 5 years old and 2 months old).

Good luck!