There were also losses for two other Premier clubs as Llandovery fell 15-18 at home to Llantwit Fardre of Two East and Banwen of Two West, the Silver Ball holders, held off Newbridge with some ease.
Cardiff were four minutes from extra time when they struck to defeat Bonymaen and Llanelli, who lost fly half Ceiron Thomas with a shoulder injury, only just came through at Bedlinog, while Swansea needed a good second-half rally to overtake Maesteg.
There were crushings wins for Cross Keys, Bridgend, Carmarthen Quins, Neath and Pontypool at home and Aberavon at Vardre, where the losers must be credited with having such a fine pitch prepared. Glamorgan Wanderers, despite having lost at Ebbw Vale in Round Three, made it to ound Five with a 23-14 win at Carmarthen Athletic - only the second Division One club to go through.
Bedwas struggled to gain a 17-7 win at Cwmllynfell and Newport won 28-3 at Pontycymmer, scoring four converted tries. The Round Five draw will be made at 4.00pm on Tuesday, January 25.
Meanwhile in Lloyds TSB Division One only one match survived and it was in the bottom half of the division as Tonmawr desperately needed a win at Fleur de Lys, but did not gain it. Fleur centre Darryl Cook scored the only home try, but three penalty goals helped to gain a 14-7 victory.
With most clubs involved in the cup, the only other scheduled game was between Narberth and Llangennech, but heavy rain rendered the Lewis Lloyd Ground unplayable.