Big Creek #4, also known as Forty Acre Lake, is a privately owned irrigation reservoir located in Mesa County, Colorado. Built in 1893, this earth dam stands at 15 feet high with a hydraulic height of 18 feet and a length of 200 feet. The reservoir has a storage capacity of 388 acre-feet and serves multiple purposes including irrigation, recreation, and water supply.

The dam, primarily constructed with stone core and resting on rock and soil foundations, underwent modifications in 1970 to enhance its hydraulic, mechanical, and structural components. With a spillway width of 15 feet and a maximum discharge capacity of 970 cubic feet per second, Big Creek #4 is classified with a low hazard potential and fair condition assessment. Despite its moderate risk rating, the dam is regularly inspected every 6 years to ensure its safety and stability.

The reservoir's emergency action plan (EAP) was last revised in 2005, and it meets regulatory guidelines. In case of a potential emergency, the dam is equipped with a slide outlet gate for controlled water release. With its picturesque surroundings and vital role in irrigation and water supply, Big Creek #4 remains a significant water resource in the region, serving both agricultural and recreational needs while adhering to state regulatory standards.