First division West Bromwich Albion, 2-1 winners over Sunderland in the third round, claimed their second Premiership scalp with a 1-0 win over Leicester at the Hawthorns today.
Former Chelsea defender Neil Clement snatched the winner, crashing home an 80th minute penalty.
In another upset, Charlton midfielder Graham Stuart missed a second-half penalty and despite scoring for the Londoners, Walsall held on for a famous victory.
The win was the best possible start for Walsall manager boss Colin Lee who took over as manager of Midlands club two days ago.
Honours were even in the London derby at Stamford Bridge where Chelsea were held to a 1-1 draw by West Ham.
Jimmy Floyd Hasselbaink's left-footed curler after 21 minutes gave Chelsea the lead only for French striker Freddie Kanoute to equalize for the Hammers seven minutes from time.
Walsall, third from bottom in the first division, upset Charlton of the Premiership 2-1 at The Valley thanks to Portuguese striker Jorge Leitao's double strike.
Everton booked their place in the fifth round with a comfortable 4-1 victory over third division Leyton Orient at Goodison Park.
Former England midfielder Paul Gascoigne rolled back the years to orchestrate Everton's win, but it was a headed own goal by Orient defender Dean Smith which gave the hosts an early lead.
Scottish international Duncan Ferguson doubled Everton's advantage before Scott Canham scored for the Londoners. Kevin Campbell, who had a loan spell at Orient 12 years ago, made it 3-1 to Walter Smith's side just before the interval.
Campbell grabbed his second goal 10 minutes to complete the rout.
England striker Andy Cole scored the decisive goal in Blackburn's 1-0 win at first division Millwall, the former Manchester United forward netting three minutes from time.
Tomorrow Arsenal will be out to avenge the 2-1 defeat to Liverpool in last year's final as they clash at Highbury.
In other matches, Newcastle face a potential banana skin at second division Peterborough, Ipswich entertain first division leaders Manchester City, Tranmere welcome Cardiff City, conquerors of Leeds United in round three, and Cheltenham host Burnley.