The R's succumbed to back-to-back defeats, as Colchester notched up their first home victory of the campaign at Layer Road.
A debut goal from Rowan Vine and a first in QPR colours from Hogan Ephraim proved scant consolation for Rangers, who remain rooted to the foot of the Championship table.
QPR trailed 3-1 at the end of a lively first period.
After Lee Camp tipped Mark Yeates' initial effort onto the bar, Mikele Leigertwood's outstretched leg diverted the ball into the R's net to hand Colchester an 18th minute lead.
The R's were level just 11 minutes later though, with Ephraim notching his first goal for the Club, when he pounced on Vine's sublime pass before slotting the ball under Dean Gerken.
Rangers switched off immediately though, and the U's took full advantage. Kemal Izzet headed past the exposed Camp to make it 2-1, before Yeates made it three in the 38th minute, firing home - via a deflection - from a dubiously awarded free-kick.
Vine gave Rangers hope when he finished with aplomb in the 58th minute for a thoroughly deserved debut goal, but the R's were their own worst enemies three minutes later, when Clive Platt stooped to head home the U's fourth.
In his first match in charge as Caretaker Manager, Mick Harford made four changes to the starting XI.
Vine and Jason Jarrett came in to make their QPR debuts, while Ephraim and Leigertwood earned recalls.
In a positional change, Leigertwood partnered Danny Cullip in defence, while Martin Rowlands occupied the right back slot.
The Rangers line-up - picked for the first time by the former Colchester Assistant Manager - included no fewer than three former U's, in the shape of Vine, Ephraim and Chris Barker.
Geraint Williams handed a full league debut to former Crystal Palace ace Danny Granville, but there was no place in the 16 for former Tottenham Hotspur marksman Teddy Sheringham.
In a lively opening, Rangers created the first opportunity of note. Ephraim's enticing in-swinging cross caused panic in the U's defence, with Dean Gerken grateful to usher the ball inches wide of his left hand post.
Buoyed by their fast start, the R's should have been in front in the seventh minute, but after Dexter Blackstock nodded down Rowlands' cross from the right flank, Vine was guilty of fluffing his lines from close range and the chance was gone.
Play switched immediately to the other end, where Kevin Lisbie nutmegged Leigertwood before pulling his left foot shot wide of Camp's left hand post.
Despite his earlier miss, Vine was seeing plenty of the ball in the opening exchanges.
His run and shot on 10 minutes was deflected wide via the boot of Adam Virgo, before the same player's cross was smashed on target by Gareth Ainsworth, who saw his shot tipped wide by Gerken.
The R's were orchestrating proceedings, so when Colchester opened the scoring on 18 minutes, Harford's men had every right to feel aggrieved.
Yeates' fearsome deflected strike was tipped onto the woodwork by Camp, but Leigertwood, facing his own goal, was powerless to prevent the ball ricocheting off his leg and into the back of a vacant net.
Vine was settling into life in a Rangers shirt fantastically well. His 25th minute effort brought the best out of Gerken, who tipped the ball to safety, and from the very next attack, his fizzing near post cross was again tipped out by the overworked U's custodian.
The equaliser the R's display thoroughly deserved arrived in the 29th minute.
The impressive Barker provided the catalyst, running from deep inside his own half before playing a neat ball into the path of Ephraim. He duly played a neat one-two with Vine, before moving the ball onto his right foot and planting a tidy finish beneath Gerken's grasp.
Rangers were on level terms for just 77 seconds though.
Danny Granville's cross was flighted to perfection and Izzet arrived bang on cue to head home from eight yards, totally wrong-footing Camp.
Colchester were rampant by now and Clive Platt should have made it three when he burst clear, only to shoot straight at Camp from 15-yards.
Gareth Ainsworth fired an ambitious volley wide a minute later, but it was Colchester who scored the all-important fourth goal of an absorbing first half in the 38th minute.
Adam Bolder appeared to win the ball when he sent the roaming Kevin Lisbie to ground and from the resultant 20-yard free-kick, Yeates drilled home his fifth of the season, albeit via the aid of a deflection.
For all their attacking exploits, Colchester were playing a dangerous game at the back, with Gerken totally miss-kicking Virgo's over-hit back pass on the stroke of half-time, only for the ball to fade a yard or so wide.
Ainsworth headed wide when he was gifted a free header in the 49th minute, before Camp made two stunning saves in as many minutes, to deny Pat Baldwin and Lisbie.
Camp's saves proved decisive and it was game on once again moments later.
Ainsworth slotted a threaded pall into the path of Vine, who steadied himself before drilling a low right foot strike into the back of the net from close range to make it 3-2.
Just when it appeared as though the R's were ready to mount a revival, Lisbie curled a shot off the post and from the resultant attack, the unmarked Platt met Yeates' cross, to head home the hosts' fourth.
Ephraim curled a soft shot on target which Gerken gathered with ease, before Jarrett seized upon Vine's flick on, only to pull his half-volley wide.
The U's had the ball in the net for a fifth time in the 73rd minute, but Lisbie's effort was ruled out for offside.
In a final throw of the dice, Harford introduced Ben Sahar at the expense of Jarrett, but it was Colchester who finished in the ascendancy, to leave Rangers contemplating back-to-back defeats, after Sunday's heavy 5-1 loss against high-flying West Bromwich Albion.
Colchester United: Gerken, Granville, Jackson, Duguid, Platt, Izzet, Yeates, Baldwin, Elokobi, Virgo, Lisbie.
Subs: Cousins, Guy, Guttridge, Connolly, Elito.
Scorers: Leigertwood o.g 18, Izzet 31, Yeates 38, Platt 64
Bookings: Duguid 67, Jackson 81
QPR: Camp, Barker, Cullip, Bolder, Blackstock, Ainsworth (Moore 86), Rowlands, Jarrett (Sahar 74), Ephraim, Vine, Leigertwood.
Subs: Cole, Bignot, Stewart.
Scorers: Ephraim 29, Vine 58
Bookings: Bolder 37, Ephraim 68, Ainsworth 84
Referee: Mr J Moss
Attendance: 5, 361
