Also on Friday night Glasgow Warriors play their second game at their new home of Scotstoun Stadium, this time Connacht are the visitors.
The westerners do not have a very good record in Glasgow and after their impressive victory over the reigning champions in the last round, the Warriors will be looking to secure their first win at their new venue.
Six teams are in action on Saturday with two games in Ireland and one in Wales, and all three home teams - Leinster, Munster and the Cardiff Blues - have a won 2, lost 1 record so far this season.
Benetton Treviso are the visitors to Wales where they face Cardiff, a team they have never beaten, at the Arms Park.
Edinburgh, who travel to Dublin to face Leinster, have not returned with a win from city since 2005, while it has been even longer since Munster's opponents, the Newport Gwent Dragons, tasted victory at Thomond Park - March 2004.
Following the tragic death of Ulster centre Nevin Spence at the weekend, the fixture between Zebre and Ulster, scheduled to be played in Parma this Friday, has been postponed.
RABODIRECT PRO12 - ROUND 4 FIXTURES:
Friday, September 21 -
Scarlets (1st) v Ospreys (11th), Parc y Scarlets, 7.05pm (live BBC 2 Wales)
Glasgow Warriors (7th) v CONNACHT (9th), Scotstoun Stadium, 7.35pm (live TG4/BBC ALBA)
Saturday, September 22 -
LEINSTER (6th) v Edinburgh (3rd), the RDS, 6.15pm (live RTÉ Two/BBC ALBA)
Cardiff Blues (5th) v Benetton Treviso (10th), Cardiff Arms Park, 6.30pm (live S4C)
MUNSTER (4th) v Newport Gwent Dragons (8th), Thomond Park, 8pm (live TG4)