The unaccounted for are not considered missing but have not contacted family or friends, Boulder County authorities Friday told KUSA-TV, Denver.
Officials in Boulder, Fort Collins and other towns warned residents to stay home, The Denver Post reported.
They suggested driving or walking could be dangerous with rain still falling and some rivers and creeks still dangerously high and could also hamper emergency workers.
"We strongly encourage people to stay home," Boulder County Sheriff Mark Beckner said in a tweet. "Dangerous to attempt to move around in city."
A wall of water swept through part of Boulder late Thursday as sirens blared, warning residents to move to higher ground.
In Denver, a man who was swept into a culvert with his dog emerged several blocks away, alive but injured, the Post said. There was no word on what happened to the dog.
The flooding began Wednesday night as record-breaking rain fell on the Rockies, sending water cascading down the mountains' eastern slopes. At least four deaths had been reported by Friday, three in Boulder and one in Colorado Springs, CNN reported.
Lyons, a small town at the junction of two creeks about 20 miles north of Boulder, was cut off by flooding Wednesday. The National Guard dispatched trucks to the town to evacuate the residents Friday.
Colorado Gov. John Hickenlooper said the evacuation began at sunrise with 21 National Guard trucks dispatched to Lyons.
Nearby Jamestown had already been evacuated, but emergency workers were unable to get to Lyons Thursday.