This rare clean sweep was all the more surprising in that it came the week after all four had lost in the Celtic League. It wasn't that form was suddenly turned on its head as some significant changes in personnel had been made across the board, but this was still an achievement to celebrate.
European Cup holders Leinster set the ball rolling with an emphatic 32-7 bonus-point victory at previously unbeaten Welsh club Scarlets. With tries by Shane Horgan, Gordon D'Arcy, Shaun Berne and Sean O'Brien, Leinster built on an emphatic 22-0 lead at the break to easily keep their opponents at bay during the second half.
Even without Ireland out-half Jonathan Sexton, who is currently on the injury list with a broken bone in his hand, Leinster moved to the top of their pool alongside London Irish and now must be high on confidence going into Saturday's return game at the RDS in Dublin.
Although there had been major doubts over Berne's ability as a playmaker, he deputised superbly for Sexton and finished with 17 points to his name. "It was a good start for Shaun, he wants to play in the team as much as anyone," said coach, Michael Cheika. "He managed the troops and his goalkicking was good. The opportunity has come now and he wants to put down a marker."
Although Cheika expressed himself disappointed with the second-half performance, he added that he didn't want "to go too heavy on the team" in advance of Saturday's match. "We certainly had a few kick-up-the-backsides warnings. Just a longer period of the better stuff, that's what we're after."
Although Leinster have still to overcome the Scarlets again, and then to take on French club Brive also at home, it's looking increasingly likely that their final pool match against London Irish in London in the new year will decide who reaches the quarter-finals.
Munster also stayed on the winning trail, but their 24-23 victory over French champions, Perpignan, at Thomond Park was much too close to a shave. Remarkably, Munster were outscored by three tries to nil and still they managed to edge the verdict and to remain at the top of their pool.
Following a disappointing start to the season during which he lost his Ireland place to Sexton, Ronan O'Gara delivered the perfect response to his critics by scoring all 24 points with seven penalties as well as superb left-footed drop goal.
However, Munster's performance was pockmarked by a host of uncharacteristic errors and so disjointed was their attacking play that they never remotely looked like scoring a try. Their new South African center, Jean de Villiers, was dropped as coach, Tony McGahan, opted for a midfield combination of Lifeimi Mafi and Keith Earls, but the backline still failed to fire.
Instead it was Perpignan who punched holes in the Munster defense with two early tries which featured some poor tackling by the home team. Then in the final minutes, the French substitute full-back, Philip Burger, ran nearly the length of the pitch for a magnificent solo try to give his side a 23-21 lead.
If Perpignan had kept their discipline they would have hung on for a famous victory and Munster would have been all but eliminated from the tournament, but Bertrand Guiry stupidly high-tackled Mafi close to the posts, and O'Gara tapped over the winning penalty for a win that was greeted with as much relief as joy.
Basically, if O'Gara had had an off-day with the boot, Munster would be down and out. As it is, they have to travel to Perpignan on Sunday where the French are nigh on invincible, so a likely defeat will see them needing convincing victories over both Treviso (away) and Northampton (home) in the new year in order to reach the knockout stages.
Ulster, meanwhile, looked to be on the back foot in their pool in advance of Stade Francais's visit to Belfast, but the home team responded superbly following a poor display last week to comprehensively defeat the French by 23-13.
Tries by Nigel Brady and Simon Danielli, along with 13 points from the inspirational Ian Humphreys, gave Ulster a second win of the campaign which leaves them level at the top of the pool with Stade.
If they are currently a long way from qualification, this unexpected win gives them an opportunity to reach the last eight for the first time in 10 years. The return game is scheduled for Brussels in Belgium - a de factor neutral venue - so Ulster are entitled to travel with more confidence than if the match was in Stade's Paris home.
While Ulster fully deserved their victory, the clash was marred by an incident involving Julien Dupuy who twice made contact off the ball with Stephen Ferris' eye area. Ferris protested to the officials who didn't see Dupuy's action, but it's likely now that the French scrum-half could be facing a lengthy ban.
"I don't want to name him, but I felt fingers in my eyes," said an incensed Ferris afterwards. "There was no apology afterwards. All he said to me on the pitch was 'I didn't do anything, nothing happened'. If someone hit me a dig on the jaw, I'd take it and get on with the game, but not when someone goes for your eyes."
Following a poor run of form in the Celtic League, there was redemption of sorts for Connacht who registered a third win in succession in the European Challenge Cup by beating English club Worcester 26-21.
If Connacht can repeat the dose in Galway on Friday, then they will be in pole position to reach the knockout stages, but coach Michael Bradley has warned against over-confidence. "It's only half-time in this battle. It's great to go over there and win, but it would be a shame to undo all that work and lose to them at the Sportsground," said Bradley.
First-half tries by Ian Keatley and the prolific Fionn Carr put them on the right track, and after a third try from recently-capped Ireland hooker, Sean Cronin, gave Connacht the lead once again, Keatley stepped up to seal the win with the last kick of the game.