Sec. 4. The salary of the Superintendent of Public Instruction of each county in Texas having a population of not less than eleven thousand and twenty-one (11,021), nor more than eleven thousand and fifty (11,050), according to the latest Federal Census, shall from and after the passage of this Act be not less than Twenty-one Hundred Dollars ($2100) per annum, nor more than Twenty-four Hundred Dollars ($2400) per annum; said salary to be set by the County Board of School Trustees of each county affected.