Chesterfield slipped back into relegation trouble when they were taken apart by a Colchester side that moved a significant step nearer to safety.
The visitors could not have expected such a comprehensive victory but the scoreline did not flatter Colchester who cruised most of the second half.
Chesterfield's performance was abject and they now face a huge game against Wycombe at home on Saturday.
Their veteran striker David Reeves described the display as diabolical and he said: "The players have let the club down, the fans down, and themselves down." Colchester took advantage after Kem Izzet had continued his impressive scoring record against the North Derbyshire club.
Izzet got both goals in a 2-0 win at Layer Road in October and he was on target again with a 43rd minute header from a Joe Keith free-kick.
On-loan striker Gareth Williams punished some slack marking to drive in a rising shot from close range in the 43rd minute and from then on Colchester did much as they pleased.
With Izzet outstanding in midfield, they pulled Chesterfield apart although the final two goals were real clangers as far as the home side were concerned.
Andy Rushbury lost the ball out on the Chesterfield left to Sam Stockley who drove in a low cross that Steve Payne turned past his own keeper in the 68th minute.
The fourth was even more farcical as substitute Dean Morgan scored 20 seconds after taking the field.
A Steve Blatherwick back header went past the advancing Ben Williams and Morgan was able to run the ball into the empty net to complete a disastrous night for Chesterfield.
It might have been different if Chris Brandon had scored with an 18-yard left-foot shot that went just wide in the 21st minute but once Izzet had put Colchester in front, the roof caved in on Chesterfield.
