‘T’ stands for Tax Slip and ‘4’ is number assigned by Canada Revenue Agency to classify Employment Income. T4 slip provides detail of your annual income and deductions. T4 slip is issued by your employer and sent to Canada Revenue Agency and a copy is provided to you. You have to report your employment income as per these T4. slips. Make sure you have them before filing taxes.
This is how a typical T4 slip looks like.