Ten-man Arsenal scraped to a 2-1 home win over League One (third division) Huddersfield on Sunday after Cesc Fabregas came off the bench to score an 86th-minute penalty and send them into the fifth round of the FA Cup.
Arsenal played just over half the match a man down after Sebastien Squillaci was sent off and also lost in-form Samir Nasri to a hamstring injury that looks likely to rule him out of next month's Champions League game against Barcelona.
Manchester City needed a goal from Edin Dzeko 10 minutes from time to earn a 1-1 draw at third division Notts County.
There was another late penalty at Molineux but Thomas Sorensen saved Nenad Milijas's effort to enable Stoke City to hang on to a 1-0 win over Wolverhampton Wanderers courtesy of an 81st-minute Robert Huth goal.
Nigeria striker Victor Obinna hit a hat-trick as West Ham United beat Nottingham Forest 3-2, while the weekend's fourth-round games are completed later on Sunday when Fulham host Tottenham Hotspur.
The fifth-round draw also took place on Sunday and produced a classic tie as Crawley, the first minor league club to reach the last 16 for 17 years, were handed a dream trip to Manchester United.
Orient were also rewarded for two upset wins with an away game against Arsenal while holders Chelsea, should they win their home replay with Everton, will host Reading.
All seemed to be going smoothly for Arsenal when Nicklas Bendtner's deflected shot put them ahead after 22 minutes. However, things changed when centre back Squillaci was sent off three minutes before halftime for a professional foul.
Huddersfield made all the running after that and, after a series of near-misses and good saves by Manuel Almunia, they levelled with a header from a corner by Alan Lee.
Arsenal had needed a stoppage-time penalty to earn a 1-1 home draw with Leeds United in the last round and were handed a similar escape route when Bendtner was shoved and Fabregas, on for the last 20 minutes, dispatched the spot kick.
Wenger was pleased to have scraped through but was more concerned with the injury to Nasri, arguably his best player this season who is now in doubt for the Champions League last 16 first-leg match against Barcelona on February 16.
"I took the gamble on Nasri - it backfired, but he could have come on and that happen as well," he said.
"He looks to have a serious hamstring injury. Usually it is three weeks, I think he will miss the Barcelona game." Manchester City, the richest club in soccer, were in danger of humiliation as Notts County, a point above the relegation zone in League One and with a squad made up predominantly of free transfers and trainees, matched their millionaire rivals in every department.
They deserved their 60th-minute lead via Neal Bishop's glancing header and were untroubled at the back until Micah Richards escaped down the right and crossed for 27 million pounds ($42.86 million) signing Dzeko to volley the equaliser.