Everton and Chelsea set for replay, Villa defeat Blackburn and Leyton Orient win at Swansea.
Salomon Kalou came off the bench to rescue a point for Chelsea at Everton.
David Moyes men had taken the lead on the half hour through a strike from former Manchester United striker Luis Saha.
Everton looked comfortable and heading through to the fifth round, until Carlo Ancelotti’s substitute Kalou struck 15 minutes from time to set up a replay at Stamford Bridge.
Aston Villa were the comfortable winners in their encounter with Blackburn Rovers.
Irish international Clark struck on 11 minutes for the home side, before Robert Pires and Nathan Delfouneso made it 3-1 before half time, after Kalinic had equalised for Rovers.
Aston Villa survived the sending off of Nathan Baker just after the half hour to keep their lead and progress to the fifth round.
The surprise of the day so far came from the Liberty Stadium, where Swansea were defeated 2-1 by Leyton Orient.
Orient went in front on 35 minutes through Jimmy Smith, before Swansea equalised right on the stroke of half time through Cedric Van Der Gun.
Both sides went for the win in the second half, and Orient snatched a dramatic victory in the 88th minute after Alan Tate put the ball in his own net.
The 3pm fixtures are currently underway.