T4 is an information slip prepared and issued by an employer to tell you and the Canada Revenue Agency (CRA) how much employment income you were paid during a tax year and the amount of income tax that was deducted. This is the slip you use when you file your income tax.
A Notice of Assessment is a statement from the Canada Revenue Agency notifying the taxpayer(you) of the amount of tax you owe, if any, the amount of tax already paid, if any, tax credits received and contributions to a Registered Retirement Savings Plan(this is the money you save for retirement). Notice of Assessment can be obtained directly from CRA.