R.J.
This is actually kind of normal. The difference between stealing and sharing at 4 is almost an invisible line. The whole "we don't take things that belong to others without asking" has to be drilled in the same way sharing, saying please, and no hitting has to be drilled in. If you don't tell her mum, and she isn't stopped each and every single time, she'll never learn. The same way a child who hits and isn't stopped will never learn. She's not being bad, she's just being a child.