Ipswich Town 1 Sheffield United 1
Dean Saunders earned United a replay with a soft goal eight minutes from time after his penalty had been saved by Richard Wright and the ball rebounded to the Welsh international.
In a frenzied final 10 minutes United full back Chris Short was sent off for kicking Mauricio Taricco and Bobby Petta needlessly fouled Marcello inside the penalty area to hand the visitors a lifeline.
Until that point Ipswich looked to be going through to the fifth round on the back of David Johnson's header in first half injury time, his 10th goal in 14 games since his £1million transfer from Bury.
Johnson, 18 goals in all this season, was a constant threat to United. After having a first half goal disallowed for offside, he later hit a post and brought a brilliant tipover save from Simon Tracey.
James Scowcroft also went close for Ipswich with a header which Short cleared from the United goalline.
Johnson was unlucky not to increase Ipswich's lead in the 59th minute when he took a throw from Mick Stockwell and cut into the United penalty area but his left foot shot rebounded from a post.
Before United had time to regroup, Johnson and Scowcroft linked up again with the latter sending a shot skimming just over the crossbar.
There was a moment of danger for Ipswich when Wright dropped Morris' corner but referee Alan Wilkie penalised Marcello for a push on the goalkeeper.
Johnson was proving a real handful for United's defence and twice within a minute midway through the second half the stocky striker went close to scoring another.
He nipped in ahead of Nicky Marker but when faced only by Tracey, lobbed the keeper but wide of the goal.
Then Tracey charged recklessly 30 yards from goal and though Johnson sped past, Marker could got back to head the shot away and save his keeper from further embarrassment.
A brilliant save by Wright prevented Marcello from levelling for United after Gus Uhlenbeek had given the ball away.
After Saunders equaliser Ipswich drove forward in an attempt to regain their lead and Johnson's shot was brilliantly tipped over by Tracey, who stood firm against mounting pressure in the closing minutes.