In addition to tonight's Ospreys v Ulster clash there are five other matches this weekend, the pick of them being tomorrow's Munster-Leinster inter-pro in Limerick.
Benetton Treviso v Connacht (tonight, 7.30pm): A 29-19 home victory over Munster in round three is Treviso's only win in this season's PRO12. But they have lost just one of their last eight games in all competitions at Stadio Monigo.
Ulster's 16-15 smash and grab act in November 2012 was the last time an Irish province won there.
Although Connacht have won only one of their last seven PRO12 matches, they are unbeaten in their last eight against Italian opponents in all competitions.
NG Dragons v Zebre (tonight, 7.15pm): Each of the Dragons' last seven PRO12 matches has been won by the side playing at home.
Since losing in Treviso last December, NGD have won their last four against Italian opponents.
Never have they managed league points in three successive matches.
Dragons have won both previous meetings.
Scarlets v Glasgow Warriors (tomorrow, 6.30pm): Scarlets have won two and lost two, though they have been victorious in their last five matches against Scottish opponents.
Glasgow – the only side with a 100% record – have won their last six PRO12 regular-season encounters and their only defeat in their last eight meetings with Welsh opponents was against Scarlets in this fixture last season.
Scarlets have lost only once to Glasgow in their last six.
Munster v Leinster (tomorrow, 6.45pm): Munster's only Thomond Park defeat since April 2012 was in this equivalent fixture last season; Leinster won 22-16.
Leinster's only loss in their past 12 matches was a 12-6 defeat in Glasgow on September 20. They have won four of their last six away against fellow-Irish provinces. Munster have lost each of their last four matches against Leinster.
If they manage a third successive victory on Munster soil, it will be Leinster's first such hat-trick since 1971.
Cardiff Blues v Edinburgh (Sunday, 2.05pm): Cardiff's only victory in their last six PRO12 contests saw them beat Connacht, 21-10 in round two. Edinburgh's only win so far came on the same night – 16-13 at home to the Dragons.
The Scots have won just one of their last 10 away – 9-7 to Zebre in Parma in March. And since December 2009, the only win in 16 visits to Wales was this game last season.