Well, in this case have you ensured that m_sfEntityBox.getVertexCount() returns at least four and called resize if that is not the case?
Yes that was the problem thanks. I set the sf::VectorArray in the constructor of the Entity, and the way I was doing it before worked, which was sloppy and looked bad so I changed it. Thanks.
Edit: For the function void Entity::setPosition() do I need to use "this" for all the variables? I would think so because I am not using the function for the Entity but for a child class. It works as is, I am just wondering.