My lease states that rent is due on the 15th of the month. The tenant and I verbally agreed to change the rent due date to the 1st. My tenant has not paid rent this month. What date should I put on the N4 notice?

Since there was no written and signed agreement between you and your tenant, you will have to use the rental period that is on your lease – specifically, the 15^th to the 14^th. Otherwise, the Landlord and Tenant Board may dismiss your application.