This weekend's fourth round of matches in the Celtic league pit the leaders of both groups against two of their biggest challengers with Leinster facing their oldest of foes, Ulster, while Neath face Group B's other unbeaten side Munster.

Leinster have looked to be continually improving as the competition has progressed and made short work of Ebbw Vale on Tuesday night - however they should face a more difficult evening at their Donnybrook ground on Friday night when Ulster come to visit. The game has added significance for both sides, as it will also be their first fixture in this season's Interprovincial Championship, the Irish route to European qualification next season. Consequently Leinster have wheeled out their big guns with Lion Brian O'Driscoll making his first start. He will be joined by several other big names, with Malcolm O'Kelly Eric Miller, Gordon D'Arcy and Shane Horgan all added to the side after missing the win over Ebbw Vale. With Leinster on the crest of a wave they will start as clear favourites against an Ulster side who have promised much but really failed to deliver. After sneaking a draw in Glasgow they were dealt a big blow by losing in the dying minutes against Llanelli - but in truth this was a fair result as, amongst other things, the line out was a shambles. Despite the return of skipper Andy Ward from injury Alan Solomons will need a major improvement if Ulster are to take the points.

The other top teams in Group A, Llanelli and Bridgend meet on Saturday evening, with the Scarlets looking to build on that narrow win in Belfast. They will have Matt Cardey back at full back and both the Easterby brothers returning to the starting line up.

At the top of Group B Neath have been confidently stringing together some fine performances in their wins over Caerphilly, Cardiff and most notably in Edinburgh during the week. However they will face their toughest task yet when they face the other unbeaten side in the Group, Munster, on Friday night.
Neath's preparation for the game has been hampered by concerns over several key players. Shane Williams is definitely still out while Allan Bateman (foot), captain Gareth Llewellyn (shoulder), Brett Sinkinson (foot) and Barry Williams (knee) are all doubts. Coach Lynn Davis is approaching the game with caution claim that Munster are: "a fine team with quality players, particularly up front. They have a well-rounded game plan and are among the favourites for the European Cup, let alone the Celtic League."
However Declan Kidney has made several changes from the team that finally overwhelmed Connacht in midweek, and will be without Alan Quinlan and Mick Galwey - with Peter Clohessy name as captain. "This week the players have two games in four days and next week it'll be three games in eight, we don't want to overplay players so for example players like Alan Quinlan and Mick Galwey get a break this time," explained Kidney.

The midweek round of matches saw first wins for both Swansea and Cardiff and both will be hoping to continue to build some momentum in round four. Swansea, still devoid of a clutch of top names were especially strong in midfield against the always combative Pontypridd and fully deserved their 11-6 win. They face Ebbw Vale, still on the ropes after their mauling in Dublin and it is hard to see the All Whites failing to gain another win. Injured hooker Garin Jenkins said before the game at Sardis Road "no league has ever been won in August" and there is no doubt that Swansea will be there or there abouts yet again next spring. Stand in captain Andy Moore was again an immense presence against Ponty and believes the side will continue to improve "We have still got a long way to go but we have kick-started our season a bit."

Cardiff will also be hoping to build on the momentum of their win over Newport when they travel to Caerphilly. The injury worries at the Arms Park are beginning to ease, with the prospect of club captain David Young and scrum half Rob Howley both close to full match fitness. For their part Newport will look to regroup against Edinburgh. Their saga over Joost continues to rumble and the mood of the fans will not have been helped by that defeat against the old enemy down the road on Wednesday. Edinburgh will have been stung by a rare home defeat by Neath on Tuesday and are notoriously bad travellers. Despite the midweek defeat a win over the Scots should be enough to virtually guarantee the black and ambers a quarter final place.