Undefeated Harlequins and Clermont can book European Cup quarterfinals this weekend with Toulon also close to securing their spot in the last eight.
English champions Harlequins, bidding to make the knock-out stages for just the fifth time, already have 19 points from four Pool Three wins and on Saturday tackle Connacht where just a losing bonus point will be enough to see them through.
But they will also be fired up by the desire to grab a home tie in the quarterfinals ahead of their final pool clash next week, a tricky trip to Biarritz.
In Pool Five, Clermont only need a home win over Exeter to book their place in the last eight for the third time in the last four seasons.
Their double triumph over two-time defending champions Leinster means that the Irish side will probably need try bonus-point victories over the Scarlets and Exeter in their last two games to stay in the race to qualify as one of two best runners-up.
Joe Schmidt's Leinster welcome rock-bottom Scarlets and will be boosted by the return of British and Irish Lions trio Brian O'Driscoll, Rob Kearney and Luke Fitzgerald.
Kearney and Fitzgerald have yet to play in the European Cup this season but both featured for an hour in the 31-16 triumph over Edinburgh in the Celtic League last week while O'Driscoll played the full 80 minutes.
Toulon, the only other undefeated team after four rounds, host Cardiff on Saturday with a five-point lead in Pool Six over Montpellier who go to struggling English side Sale.
But the Top 14 leaders suffered a jolt to their confidence last weekend when they slumped to a 19-15 defeat to Racing Metro.
That loss had Toulon president Mourad Boudjellal fretting over the strength of his squad despite huge investment in a wealth of overseas talent.
"When we are missing one or two important players, we are not the same team," said Boudjellal.
Two-time winners Leicester and four-time champions Toulouse are locked in a battle of European war horses in Pool Two.
Leicester have a one-point advantage over their French rivals, who are bidding to make the knockout stages for the 15th time.
They meet at Leicester's Welford Road in the final round of group games next week.
On Sunday, however, the Tigers go to Swansea to face the Ospreys while Toulouse welcome bottom-club Treviso who have just one point from four games.
In Pool One, Saracens are two points clear of Racing Metro ahead of their match against the French side which is taking place in Nantes.
A win for Saracens would end Racing's hopes of automatic qualification while Munster, the 2006 and 2008 champions, will stay in contention if they beat pointless Edinburgh.
Ulster, the 2012 runners-up, lead Pool Four, are three points ahead of Castres, and meet Glasgow at Ravenhill on Friday.
Ulster suffered their first defeat of the season when Northampton won 10-9 in Belfast before Christmas.
Round Four fixtures and teams:
Friday, January 11
Northampton Saints v Castres Olympique
(Franklin's Gardens - Kick-off: 20.00, 20.00 GMT)
Teams:
Northampton: 15 Ben Foden, 14 James Wilson, 13 George Pisi, 12 Dom Waldouck, 11 Jamie Elliott, 10 Stephen Myler, 9 Lee Dickson, 8 Phil Dowson, 7 Tom Wood, 6 Calum Clark, 5 Courtney Lawes, 4 Samu Manoa, 3 Paul Doran-Jones, 2 Dylan Hartley (captain), 1 Soane Tonga'uiha.
Replacements: 16 Mike Haywood, 17 Alex Waller, 18 Tom Mercey, 19 Mark Sorenson, 20 GJ Van Velze, 21 Martin Roberts, 22 Ryan Lamb, 23 Tom May.
Castres: 15 Brice Dulin, 14 Max Evans, 13 Romain Cabannes, 12 Rémi Lamerat, 11 Marcel Garvey, 10 Daniel Kirkpatrick, 9 Rory Kockott, 8 Antonie Claassen, 7 Yannick Caballero, 6 Jannie Bornman, 5 Iosefa Tekori, 4 Christophe Samson, 3 Anton Peikrishvili, 2 Mathieu Bonello, 1 Yannick Forestier.
Replacements: 16 Brice Mach, 17 Saimone Taumoepeau, 18 Michaël Coetzee, 19 Matthias Rolland, 20 Paul Bonnefond, 21 Pedrie Wannenburg, 22 Pierre Bernard, 23 Thomas Sanchou.
Referee: George Clancy (Ireland)
Assistant referees: Paul Haycock (Ireland), Michael Black (Ireland)
TMO: Peter Ferguson (Ireland)
Ulster v Glasgow Warriors
(Ravenhill - Kick-off: 20.00, 20.00 GMT)
Teams:
Ulster: 15 Jared Payne, 14 Andrew Trimble, 13 Darren Cave, 12 Paddy Wallace, 11 Craig Gilroy, 10 Paddy Jackson, 9 Ruan Pienaar, 8 Nick Williams, 7 Chris Henry (captain), 6 Robbie Diack, 5 Iain Henderson, 4 Louis Stevenson, 3 John Afoa, 2 Rory Best, 1 Tom Court.
Replacements: 16 Rob Herring, 17 Calum Black, 18 Declan Fitzpatrick, 19 Neil McComb, 20 Roger Wilson, 21 Paul Marshall, 22 Michael Allen, 23 Chris Cochrane.
Glasgow: 15 Peter Murchie, 14 Sean Maitland, 13 Alex Dunbar, 12 Peter Horne, 11 DTH van der Merwe, 10 Duncan Weir, 9 Henry Pyrgos, 8 Josh Strauss, 7 Robert Harley, 6 James Eddie, 5 Al Kellock (captain), 4 Tom Ryder, 3 Moray Low, 2 Doug Hall, 1 Ryan Grant.
Replacements: 16 Pat MacArthur, 17 Gordon Reid, 18 tbc, 19 Nick Campbell, 20 Tim Swinson, 21 Nikola Matawalu, 22 Scott Wight, 23 Stuart Hogg.
Referee: Pascal Gauzere (France)
Assistant referees: Laurent Cardona (France), Stéphane Pomarède (France)
TMO: Hervé Dubes (France)
Sale Sharks v Montpellier
(Salford City Stadium - Kick-off: 20.00, 20.00 GMT)
Teams:
Sale Sharks: 15 Cameron Shepherd, 14 Tom Brady, 13 Mark Cueto, 12 Mark Jennings, 11 Charlie Amesbury, 10 Danny Cipriani, 9 Will Cliff, 8 Josh Beaumont, 7 David Seymour, 6 James Doyle, 5 Tom Holmes, 4 Richie Gray, 3 Vadim Cobilas, 2 Marc Jones, 1 Aston Croall.
Replacements: 16 Tom Cruse, 17 Ross Harrison, 18 Tony Buckley, 19 Kearnan Myall, 20 Tommy Taylor, 21 Nathan Fowles, 22 Rob Miller, 23 Jordan Davies.
Montpellier: 15 Pierre Bérard, 14 Timoci Nagusa, 13 Thomas Combezou, 12 Santiago Fernandez, 11 Yohann Artru, 10 Francois Trinh-Duc, 9 Benoit Paillaugue, 8 Johnnie Beattie, 7 Mamuka Gorgodze, 6 Fulgence Ouedraogo, 5 Aliki Fakate, 4 Mickael Demarco, 3 Maximiliano Bustos, 2 Agustin Creevy, 1 Mikheil Nariashvili.
Replacements: 16 Rassie van Vuuren, 17 Yvan Watremez, 18 Barry Fa'amausili, 19 Rémy Martin, 20 Alexandre Bias, 21 Eric Escande, 22 Paul Bosch, 23 Lucas Amorosino.
Referee: Leighton Hodges (Wales)
Assistant referees: Rhys Thomas (Wales), Wayne Davies (Wales)
Saturday, January 12
Harlequins v Connacht
(Twickenham Stoop - Kick-off: 13.35, 13.35)
Teams:
Harlequins: TBC
Connacht: TBC
Referee: Mathieu Raynal (France)
Assistant referees: Jean-Luc Rebollal (France), Mourad Zitouni (France)
TMO: John Sheehan (Ireland)
Zebre v Biarritz
(Stadio XXV Aprile - Kick-off: 14.35, 13.35 GMT)
Teams:
Zebre: TBC
Biarritz: TBC
Referee: Peter Fitzgibbon (Ireland)
Assistant referees: Stuart Gaffikin (Ireland), David Keane (Ireland)
TMO: Dermot Moloney (Ireland)
Toulon v Cardiff Blues
(Stade Felix Mayol - Kick-off: 14.35, 13.35 GMT)
Teams:
Toulon: TBC
Cardiff Blues: TBC
Referee: JP Doyle (England)
Assistant referees: Tim Wigglesworth (England), Andrew Pearce (England)
TMO: David Grashoff (England)
Racing Metro v Saracens
(La Beaujoire - Kick-off: 16.40, 15.40)
Teams:
Racing Metro: TBC
Saracens: TBC
Referee: Alain Rolland (Ireland)
Assistant referees: Mark Patton (Ireland), Brian Macneice (Ireland)
TMO: Simon McDowell (Ireland)
Leinster v Scarlets
(RDS - Kick-off: 18.00, 18.00 GMT)
Teams:
Leinster: TBC
Scarlets: TBC
Referee: Jérôme Garces (France)
Assistant referees: Sébastien Cloute (France), Alexandre Ruiz (France)
TMO: Jean-Claude Levrier (France)
Clermont Auvergne v Exeter Chiefs
(Stade Marcel Michelin - Kick-off: 19.00, 18.00 GMT)
Teams:
Clermont Auvergne: TBC
Exeter Chiefs: TBC
Referee: Nigel Owens (Wales)
Assistant referees: Neil Hennessy (Wales), Jon Mason (Wales)
TMO: Derek Bevan (Wales)
Sunday, January 13
Edinburgh v Munster
(Murrayfield - Kick-off: 12.45, 12.45)
Teams:
Edinburgh: TBC
Munster: TBC
Referee: Romain Poite (France)
Assistant referees: Cyril Lafon (France), Stéphane Boyer (France)
TMO: Daniel Gillet (France)
Ospreys v Leicester Tigers
(Liberty Stadium - Kick-off: 15.00, 15.00 GMT)
Teams:
Ospreys: TBC
Leicester Tigers: TBC
Referee: John Lacey (Ireland)
Assistant referees: Sean Gallagher (Ireland), Kevin Beggs (Ireland)
TMO: Marshall Kilgore (Ireland)
Toulouse v Treviso
(Stade Ernest Wallon - Kick-off: 16.00, 15.00 GMT)
Teams:
Toulouse: TBC
Treviso: TBC
Referee: Wayne Barnes (England)
Assistant referees: Greg Garner (England), Robin Goodliffe (England)
TMO: Graham Hughes (England)
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