Many items may be included on a T4. Employment income, taxable benefits, income eligible for EI and CPP, payroll deductions paid by the employer (i.e. EI and CPP), tax deducted, pensions, etc.
In most cases, T4s are prepared correctly. However, be careful and review them. Is there something you would expect but not there? Your tax preparer may not be able to catch T4 mistakes as the preparer may not have all the information concerning your employment. Some mistakes include missing CPP-eligible income, EI-eligible income, and missing car allowances.