It is the responsibility of your employer to provide you with a T4 by the end of February.
You can estimate or use your pay stubs. Be prepared that their might be some adjustments if CRA gets the T4. You should call CRA as well and file an employee complaint form.
Contact CRA at 1-800-959-8281 for more information. When you call, make sure you have last year's return and personal information on hand because they'll need to verify your account.