Two clubs could clinch automatic quarter-final qualification in Round 5 - Harlequins and ASM Clermont Auvergne - while Toulon, the only other club with four from four so far, need just six more match points to take the Pool 6 honours.
But for others prestigious quarter-final qualification is on a knife edge and is set to go right down to the wire with hopes staying alive or dying in crucial Round 5 contests.
Pool 1
One of the tightest of the six Pools with just three points separating the top three sides. But Saracens have their quarter-final destiny in their own hands. They are two points clear of Racing Metro 92 with their Round 5 showdown in Nantes at Stade de la Beaujoire now the key contest of the Pool. A regulation win for Saracens, denying Racing anything from the match, would rule the French club out of the running for automatic qualification. But Munster will stay right in the mix if they beat Edinburgh Rugby ahead of their home tie against Racing in Round 6.
Pool 2
Another tight Pool with any remaining try bonus points potentially crucial. Leicester Tigers are a point ahead of Toulouse - who they meet at Welford Road in Round 6 - while the Ospreys can still have a major influence on events with the Tigers having to go to the Liberty Stadium in Round 5 while Toulouse have home advantage against Benetton Treviso.
Pool 3
It's Harlequins' to lose. The English champions need just a losing bonus point at home to Connacht Rugby in Round 5 to book their quarter-final place with a match to spare. However, home advantage in the last eight is the next prize and their trip to second-placed Biarritz Olympique in Round 6 remains a major target for Conor O'Shea's squad, including this season's joint leading try scorer Danny Care.
Pool 4
All to play for, although Ulster Rugby, the 2012 finalists, are in the box seat three points clear of Castres Olympique. Glasgow Warriors are the Round 5 visitors to Ravenhill while Castres' hopes of progress will be on the line against Northampton Saints at Franklin's Gardens. The Saints are five points adrift of Ulster but both will be wary of their away trips in Round 6 to Glasgow and Castres respectively.
Pool 5
ASM Clermont Auvergne require nothing more than a regulation four-point win at home to Exeter Chiefs to book their place in the last eight for the third time in the last four tournaments. Their double over Leinster means that the defending champions almost certainly need try bonus-point victories over the Scarlets and Exeter to be in the best runners-up shakedown. However, there are already four other second-placed clubs with more points than Leinster with just two places up for grabs.
Pool 6
Toulon are four from four so far and have a five-point cushion over Montpellier. Toulon host Cardiff Blues in Round 5 - the Welsh are still winless - while their French rivals travel to Salford City Stadium to take on Sale Sharks knowing a win is vital to keep them in the hunt and leave their all-French contest in Round 6 with plenty at stake.
