A double strike from Scott Parker either side of half-time and one each for Claus Jensen and Radostin Kishishev made it 14 matches without a league victory for Glenn Roeder's men, who once again contributed to their own downfall with a poor defensive display that meant our two strikes - both own goals credited to Richard Rufus and Mark Fish - counted for nothing in the end.
It was a hugely disappointing evening for Hammers, who had arrived with plenty of optimism following yesterday's signing of Les Ferdinand. The 36-year-old striker was thrown straight in for his debut and provided some much-needed presence, both in attack and in his own penalty area, where he was ironically our most effective defender at times, winning several headers
Roeder was also able to include Michael Carrick, who had missed Sunday's defeat at Arsenal with a groin strain, but the Hammers boss was without Joe Cole, forced to withdraw due to a calf problem that is likely to keep him out of our FA Cup fourth round clash against Manchester United on Sunday but will hopefully have healed by the time Blackburn Rovers visit Upton Park next Wednesday in what is another extremely vital Premiership clash.
This trip to South London was also vital, though, and generally considered to be a 'must win' fixture for Roeder's team. After the eight-goal thriller between the two sides in last season's equivalent fixture, both sets of supporters were treated to another pulsating London derby, packed with action, excitement and controversy from the first minute to the last.
With no less than four ex-Charlton players in the Hammers' starting line-up - Scott Minto, Lee Bowyer and trainees Jermain Defoe and Gary Breen - there was further spice added to the fierce local rivalry that already exists between the two clubs.
The swirling wind and torrential rain also played its part in a frenetic and scrappy first half, with Bowyer - no doubt fired up by the abuse thrown at him by his former fans - displaying a particularly enthusiastic approach to the early exchanges. After several full-blooded encounters, the 25-year-old found himself in the book after 14 minutes when he reacted to a tackle from Parker, who was also shown the yellow card.
The pace and commitment didn't let up, though, and both Ferdinand and Christian Dailly were left with blood-stained shirts after typically brave aerial challenges in the first half.
Charlton had made the livelier start to proceedings, when Euell collected a loose ball on the right after just two minutes and raced into the penalty area before firing a cross-shot wide of the far post.
Five minutes later, Hammers were let off the hook again, as Jensen's corner from the left wasn't dealt with and Bartlett was handed a glorious chance to open the scoring but ballooned his acrobatic volley high over the bar.
With the hosts clearly on top, it was certainly a surprise when Hammers took the lead on 19 minutes with their first real attack of the game. Bowyer's clever pass out of defence led to a flowing move involving Ferdinand, Carrick and Cisse that culminated in the Frenchman challenging Rufus, who appeared to get the final touch as the ball looped up and over Kiely into the net.
The goal understandably shocked Alan Curbishley's men, and the visitors enjoyed a spell of domination that could have resulted in them doubling their tally. Cisse fired a volley over the bar from 18 yards, while Minto's effort from the edge of the area took a deflection that sent it wide of the target.
At the other end, though, Hammers were looking increasingly fragile against the three-pronged attack of Euell, Bartlett and Lisbie, and the hosts fired a warning in the 38th minute when Fortune escaped his marker at a corner to head just inches wide of the target.
Four minutes later, Charlton were level. Dailly was penalised for needlessly climbing on Bartlett some 30 yards out and Jensen stepped up to curl a magnificent free-kick past the despairing dive of James and into the top right hand corner of the net.
If that was a blow, worse was to follow for Hammers as, deep into injury time at the end of the half, the Addicks took the lead. Euell picked up the ball on the left and threaded a pass into the area that Bartlett cleverly stepped over, allowing Parker time and space to volley past James.
The interval came as a welcome relief for the visitors, allowing them time to regroup and halt Charlton's momentum but, just seven minutes into the second half, Parker struck a killer third goal that once again owed more to the slack marking of the Hammers defence.
A deep free-kick from Jensen was only half-cleared and, after Fish completely missed the ball on the penalty spot, Parker was left on his own to fire a low left-foot shot from the edge of the area that flew past an unsighted James and into the bottom corner of the net.
From being 1-0 up to 3-1 down in the space of 11 minutes, Hammers were clearly shell-shocked, but refused to throw in the towel and, after Breen and Bowyer had fired efforts wide, they pulled themselves back into the contest with a second goal on 62 minutes.
Ferdinand was again involved in the build-up, holding the ball up well before laying it back to Carrick, who fired in a low shot from 25 yards out that appeared to be heading wide until it struck Fish and flew past the wrong-footed Kiely.
Roeder then introduced young defender Glen Johnson for his first team debut as a substitute for the tiring Cisse. The 18-year-old slotted in comfortably at right-back and gave a good enough account of himself to suggest that he may be involved more often between now and the end of the season.
With half an hour still to play, Hammers had enough time to search for all three points but, as they toiled and grafted without showing any real quality or conviction in the final third, Charlton's impressive central defensive trio of Fish, Rufus and Fortune dealt easily with everything thrown at them.
With just four minutes left, Hammers were presented with the chance they had been hoping for, when Sinclair latched on to a loose ball and skipped into the box, but the England midfielder toe-poked his effort wide before claiming he had been tugged by a defender as he was about to shoot.
Referee Eddie Wolstenholme - who at times struggled to keep up with the frantic pace of the game - waved away the penalty appeals and, to add insult to injury, Charlton grabbed a fourth goal in the final minute.
With Hammers committing men forward, Bowyer tried to do too much in his own half and lost possession to substitute Johansson, who raced down the right and sent over a cross that was volleyed goalwards by Svensson. The Swede's effort was saved by James, but Bulgarian defender Kishishev was on hand to stab home the rebound - his first goal for Charlton.
While the fourth made it look slightly more comfortable for the Addicks than it actually was, there is no denying that this is another major setback for Roeder and his team as they attempt to fight their way out of the bottom three.
The focus now turns to Sunday's cup tie at Old Trafford, but next Wednesday's visit of Blackburn is ultimately a more important match. Glenn is hoping to have first choice strike pairing Paolo Di Canio and Fredi Kanoute back by then, but the Hammers boss will also now be hoping that his search for defensive improvements also comes to fruition in the next seven days.