Scotland's capital team, Edinburgh, are also in action on Friday night as they welcome the Scarlets to Murrayfield. Neither team has made the start to the season they would have wished for - both will be searching for their second victory of the campaign.
There are two games in Ireland on Friday night, Leinster hosting the Cardiff Blues and Ulster facing Benetton Treviso.
Leinster succumbed to early season leaders Glasgow at Scotstoun Stadium last weekend, but their record in Dublin is very impressive. Cardiff will be breaking new ground if they come back from the RDS with a win.
Ulster saw their own home winning run bought to an end by Glasgow last time out, but a timely victory over Treviso will help to reinforce Ravenhill's reputation as a formidable place to play.
The two remaining Irish provinces are both in action on Saturday evening, and both against Welsh opposition.
First up Connacht welcome the Ospreys to the Sportsground in Galway. The Ospreys have recorded the only draw of the season so far when they were drew 29-all with Leinster at the RDS in round 2, and are the only other unbeaten team so far in this campaign.
The round wraps up in Cork where Munster entertain the Newport Gwent Dragons. This game sees third in the table host fourth, Munster sitting higher than the Dragons courtesy of securing more try-scoring bonus points than their opponents.
RABODIRECT PRO12 - ROUND 4 FIXTURES:
Friday, September 27 -
ULSTER (6th) v Benetton Treviso (8th), Ravenhill, 7.05pm (live BBC 2 Northern Ireland)
Zebre (11th) v Glasgow Warriors (1st), Stadio XXV Aprile, 7.30pm local time (live Mediaset/BBC ALBA)
LEINSTER (5th) v Cardiff Blues (7th), the RDS, 7.35pm (live TG4/BBC 2 Wales)
Edinburgh (12th) v Scarlets (9th), Murrayfield, 7.45pm
Saturday, September 28 -
CONNACHT (10th) v Ospreys (2nd), the Sportsground, 5pm
MUNSTER (3rd) v Newport Gwent Dragons (4th), Musgrave Park, 6.45pm (live www.rte.ie/S4C/full deferred coverage RTÉ Two)