The Spaniard swept home to secure a replay as Chelsea, who missed out on one Wembley appearance after losing over two legs to Swansea City in the Capital One (League) Cup semi-finals, twice needed to come from behind at Griffin Park.
Chelsea, who have lifted the trophy in four of the last six seasons, were second-best throughout the first half and went behind just before halftime when Italian Marcello Trotta lashed home.
Brazilian Oscar equalised with some fine individual skill 10 minutes into the second half but the hosts stunned their London rivals by going ahead again with Harry Forrester's penalty on 73 minutes.
In later games, Championship (second division) Leeds United host Tottenham Hotspur at Elland Road in a 1400 GMT kickoff before third tier Oldham Athletic take on Liverpool at 1600.
The draw for the fifth round is made at 1745.
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- Swansea City