1) Be confident with yourself. You must be good, otherwise any Big 4 would not have phoned you for an interview. Plus, usually, Big 4s are only recruiting from fresh graudates or co-ops for entry-level audit positions. They must really like you. And you must have outstanding working experiences and qualifications!!
2) I hope you are still single, or married without any kids yet. Otherwise, even if Big 4 wants to hire you, you need to give it a second thought. In the busy season, life is xxxxx.. When you get there, you will know what it means.