If you did not receive a T4 from one of your employers you can:
1. Contact the employer and ask them for a duplicate copy of your T4
2. If you have a MyCRA account, go to your account and try to see if there is a copy of your T4 there (one copy of the T4 issued by your employer is also sent to CRA).
3. If no T4 was issued by your employer and you can't get them to give you one, you should contact the payroll division at CRA and tell them that you are unable to get a T4 from your employer and they will advise you as to your options.