4. (1) Where a payment in respect of a bonus or retroactive increase in remuneration is made by an employer to an employee whose total remuneration from the employer, including the bonus or retroactive increase, may reasonably be expected not to exceed $5,000 in the taxation year of the employee in which the payment is made, the employer shall deduct or withhold in lieu of the amount determined under section 3, in the case of an employee who reports for work at an establishment of the employer in Ontario, an amount equal to the difference between 15 per cent of the payment and the amount of the payment determined under subsection 103 (1) of the Federal Regulations.  R.R.O. 1990, Reg. 646, s. 4 (1). (2) Where a payment in respect of a bonus is made by an employer to an employee whose total remuneration from the employer, including the bonus, may reasonably be expected to exceed $5,000 in the taxation year in which the payment is made, the amount to be deducted or withheld therefrom by the employer is, (a) the amount determined under section 3 in respect of an assumed remuneration equal to the aggregate of, (i) the amount of regular remuneration paid by the employer to the employee in the pay period in which the remuneration is paid, and (ii) an amount equal to the bonus payment divided by the number of pay periods in the taxation year of the employee in which the payment is made, minus, (b) the amount determined under section 3 in respect of regular remuneration paid by the employer to the employee in the pay period, multiplied by, (c) the number of pay periods in the taxation year of the employee in which the payment is made.  R.R.O. 1990, Reg. 646, s. 4 (2). (3) Where a payment in respect of a retroactive increase in remuneration is made by an employer to an employee whose total remuneration from the employer, including the retroactive increase, may reasonably be expected to exceed $5,000 in the taxation year of the employee in which the payment is made, the amount to be deducted or withheld therefrom by the employer is, (a) the amount determined under section 3 in respect of the new rate of remuneration, minus, (b) the amount determined under section 3 in respect of the previous rate of remuneration, multiplied by, (c) the number of pay periods in respect of which the increase in remuneration is retroactive.  R.R.O. 1990, Reg. 646, s. 4 (3). (4) Subject to subsection (5) and in lieu of the amount determined under section 3, where a payment described in subsection 103 (6) of the Federal Regulations is made by an employer to an employee who is a resident of Canada and who reports for work at an establishment of the employer in Ontario, the employer shall deduct or withhold therefrom, (a) if the payment does not exceed $5,000, the amount by which 10 per cent of the payment exceeds the amount that would be deducted or withheld from the payment under paragraph 103 (4) (a) of the Federal Regulations; (b) if the payment exceeds $5,000 but does not exceed $15,000, the amount by which 20 per cent of the payment exceeds the amount that would be deducted and withheld from the payment under paragraph 103 (4) (b) of the Federal Regulations; and (c) if the payment exceeds $15,000, the amount by which 30 per cent of the payment exceeds the amount that would be deducted and withheld from the payment under paragraph 103 (4) (c) of the Federal Regulations.  R.R.O. 1990, Reg. 646, s. 4 (4). (5) Where the payment referred to in subsection (4) is a payment referred to in subsection 103 (5) of the Federal Regulations, subsection 103 (5) of the Federal Regulations applies for the purposes of subsection (4).  R.R.O. 1990, Reg. 646, s. 4 (5).