Lesson four breaks down some fallacies of AI: we will look at the previous artificial intelligence winters - a period of decrease funding and interest in the field. Delving into the core of Explainable AI(XAI) will be the focus of this lesson; discovering how biases are represented throughout the creation of a model and how these undesirable biases are being addressed with XAI. Some of the concepts will include over- and underfitting; and the bias-variance trade-off.