Usually a difference of 4,000 to 5,000 is not a problem. Sometimes a W2 might not exactly match your paystub if you have pre-tax deductions like health insurance, retirement if they are non-taxable. Now, if you have a substantial salary differnce like 10,000 or more, USCIS might question the company's ability to pay from the priority date(which is the date you filed your labor) to the current.