When my bio son was 4ish he had many imaginary friends. His favorites were Wood-Pood, Jackie (a boy) and Flower. He also had his "Hand-Friends," he pretended his friends were people and talked to them.
I am cracking up writing this. Anyhow, he is 12 now, smart & healthy as a whip, and still has a very rich inner life.
My neice (3ish) had an imaginary friend for a while too, who constantly got in trouble. I think that it was a good way to vent her own frustration.
Though both ds & my neice have siblings now, they didn't at the time. I don't know if that has anything to do with it or not, but I personally think it is very healthy & fine.
Peace,
Amy