Challenging 4 year old

[wallplug789]

Hi, I was looking for some advice really. My step-daughter has two daughters, a 7 year old and a 4 year old. My concern with with little one. She's at pre-school, and everything fine, but she's so naughty at home. An example is yesterday, we were at her house, having a BBQ, the two girls were just being 7 & 4 year olds. The next thing we know is that the eldest child comes to her mother and said that the little one, has just poo-ed and wee-d on the front step. She knows where she's supposed to go, and does regularly, she just plays up dreadfully. Do you think it's just attention?
Thanks in advance, from a step-granny.