4. Employer's liability to deduct and pay tax on behalf of employees.- The tax payable under this Act by any person earning a salary or wage, shall be deducted by his employer from the salary or wage payable to such person before such salary or wage is paid to him, and such employer shall, irrespective of whether such deduction has been made or not when the salary or wage is paid to such persons, be liable to pay tax on behalf of all such persons:

Provided that, if the employer is an officer of Government, the State Government may, notwithstanding anything contained in this Act, prescribe by rules the manner in which such employer shall discharge the said liability