Unbeaten Munster need a bonus point victory at home to Castres Olympique to guarantee top spot. A bonus point triumph would take them to 23 points, which is the most the Scarlets in second place can achieve over the final two rounds.
Munster have beaten the Scarlets in both of their pool matches and would therefore go ahead of them on the head-to-head basis.
A straightforward win for Munster, and any form of defeat for the Scarlets this weekend, would also guarantee a 13th quarter-final for the province.
Wide open with Edinburgh and the Cardiff Blues neck and neck on 13 points and London Irish still in contention with matches against the top two to come.
Any kind of win for Leinster over Glasgow Warriors at Firhill Stadium will be enough to guarantee the reigning champions safe passage into the last-eight.
The only other unbeaten team in this season's tournament along with Munster, they opened with a draw in Montpellier - Leo Cullen and his team-mates have a six-point lead over their Scottish hosts going into round 5.
Too close to call with three major sides still in the hunt. Ulster go into the final two rounds with a two-point lead over Leicester Tigers against whom they incurred their only defeat in the pool to date.
Tigers have Clermont Auvergne one point behind them on 11 points. Ravenhill will host one of the key battles in round 5 when Ulster meet the Tigers, while Clermont Auvergne travel to Aironi Rugby before hosting Ulster in round 6.
Saracens could win their pool with a game to go if they can clinch a bonus point victory over Biarritz Olympique at Vicarage Road and prevent the French club from earning a point.
That would take Saracens to 19 points and a guaranteed place in the quarter-finals.
However, an away win for Biarritz would open up the pool for a mad scramble in round 6 when the Ospreys travel to France and Saracens head to Italy to face Benetton Treviso.
The battle between four-times champions Toulouse and reigning Amlin Challenge Cup holders Harlequins looks destined to run to round 6.
Toulouse lead 'Quins by a point in the head-to-head stakes over their two pool matches and also lead the pool by a point. They host Connacht in round 5 before heading to Gloucester in round 6.
This weekend 'Quins host Gloucester, who are still in the mix with nine points in third place, before ending with a trip to Galway to face Connacht.
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