Employees in Work Week Group 1 or Work Week Subgroup 4A may be compensated for compensable overtime by cash compensation. Employees in Work Week Subgroups 4B and 4D, except upon movement between agencies may not receive cash compensation for compensable overtime unless authorized in advance by the Department or sections 599.703 and 599.710 of these regulations. Unless otherwise specified by the Department pursuant to section 599.703 of these regulations, the rate of cash compensation paid for compensable overtime shall be time and one-half the hourly equivalent of the employee's monthly salary as of the time the overtime was worked, which shall be determined as provided in section 599.670 of these regulations, except that in no event shall the rate of compensation for Work Week Group 4 employees exceed the hourly rate established by the Department.