The Colorado State Legislature Size Amendment, also known as Measure 4, was on the November 8, 1966 ballot in Colorado as an initiated constitutional amendment, where it was approved. The measure provided for a 35 member Senate and a 65 member House of Representatives.
| Colorado Issue 4 (1966)|
| Yes|| 374,884|| 70.34%|
Election Results via: The Colorado Legislative Council
Text of measure
The language appeared on the ballot as:
|| An Act to amend Article V of the State Constitution, providing for a Senate of not more than thirty-five members and a House of Representatives of not more than sixty-five members; provides for single member districts in both Houses, each district in each House to be substantially equal in population; provides standards for formation of districts; provides for revision of districts by the General Assembly in 1967 and after each decennial census thereafter, under penalty of loss of compensation and eligibility of members to succeed themselves in office; and includes a severability and savings clause.