Skipper Matthew Tucker scored the only try in the 80 minutes that was converted by Gareth Davies, who also added two penalties that looked to have given Penallta a famous win over opponents two divisions above them in the league pyramid.
Yet Narberth, third in Division One West, finally flexed their muscles to score twice in injury time through Sean Brace and Andrew Davies to win 19-13 and break Penallta hearts in the cruellest fashion.
To make matters worse for Penallta, time had been stopped for several minutes while referee Wayne Davies received treatment after being struck in the face by the ball.
Abercynon of Division One (East) also had a narrow escape after surviving a gutsy display from Fourth Division Pwllheli before holding out 29-24.
Tryfan Edwards and Bleddyn Davies scored Pwllheli's tries, converted by Dylan Llyr who also added three penalties and a conversion. Yet two scores from wing Daniel Hitchings and tries from Chris Keepings and Gethin King saw Abercynon through to the fifth round.
Nantyfyllon gained some revenge for the Division Three teams with a 15-5 triumph over Ystrad Rhondda from the First Division following tries by Gary Davies, Dean Edwards and David Pearce.
Pontypool, cup winners in 1983, struggled before seeing off Division Three (South-West) high flyers Heol-y-Cyw 20-17 thanks to tries from Dave Smith and Darren Pearce.
Division One (East) leaders Newbridge held out 24-11 against Llantrisant at the Welfare Ground while their counterparts from the west, Bonymaen, had an easier time at Tredegar from where they returned 32-5 winners following a double from wing Mark Harvey.
Carmarthen Quins won the all Division One (West) clash against Merthyr 22-13, while Cwmllynfell edged past Bargoed 15-7.
Beddau cruised into the next round with a 59-3 demolition of Caernarfon of Division Four. Bridgend Athletic kept their cup hopes alive with a 26-17 win at Fleur De Lys, St Peter's won the Cardiff derby against Llandaff North 39-15 but Division Three (South-West) title candidates Mumbles went down 39-5 at Llangennech.
Llanishen shut-out Newport Saracens 25-0, Loughor ran in six tries in a 40-14 win over Ynysybwl while Caerphilly, finalists in 2004, won 32-10 at Maesteg Celtic. Whitland won 33-21 at Penclawdd, Seven Sisters sent Nantyglo packing 30-10 and Tonmawr completed the round with a 28-8 win over Pencoed.