@smorgs,
She is only 4 months old, smorgs.
She will get better at doing her poo outside as she gets older, I'm certain.
In the meantime, keep doing what you're doing to teach her.
A thought: is the area (outside) you want her to poo in
conducive to a small kitten's needs? Like loose, sandy soil which is easy for a kitten to turn over? If not, you might consider creating a sandy "poo area" that she might find more attractive.