The Foxes came up trumps in a five-goal affair with Millwall to make it this far, while Tony Mowbray's men required a replay and late drama to get the better of Hull City in the third round.
Match preview
Prior to being summoned back to parent club Chelsea, Italian starlet Cesare Casadei stole the show in the first half of Leicester's FA Cup third-round encounter with Millwall at The Den, heading in from a Marc Albrighton delivery before teeing up Ricardo Pereira for a warranted second.
Tom Cannon proceeded to add a third for Leicester, latching onto a magnificent long pass from Callum Doyle, but only after Duncan Watmore had given Millwall a lifeline, and the deficit was cut back down to one again when Zian Flemming converted a well-worked corner routine.
Flemming's 86th-minute strike came too late to save Millwall's bacon, though, as Leicester reached round four of the FA Cup for the fifth year running and recorded their 16th win in the competition since the start of the 2017-18 season; only Manchester United, Manchester City and Chelsea have more triumphs under their belt in that time.
However, the beating of Millwall was not an immediate sign of things to come for Leicester, who have since accrued just one point from their last two Championship affairs, suffering a surprise 3-1 loss to Coventry City before blowing a 1-0 lead in a two-goal draw with promotion rivals Ipswich Town.
The destiny of the Championship title is certainly not a foregone conclusion just yet, but Enzo Maresca's team boast an unassailable seven-point lead over the Tractor Boys with 28 games played, and Tuesday's stalemate did at least stretch the hosts' unbeaten streak at the King Power Stadium to a respectable six matches.
Likewise, a one-goal victory over Championship counterparts was all that Birmingham City required to advance in the FA Cup, although both Blues and Koji Miyoshi needed two bites at the cherry to eliminate Hull City at the third-round stage.
Ten days on from a tense 1-1 draw on Hull's turf, the Tigers silenced a sparsely-populated St Andrew's through Jason Lokilo's 16th-minute finish, but Jay Stansfield restored parity for Mowbray's men before Miyoshi's injury-time winner, which he inadvertently set himself up for with an initial miskick.
By sending the Tigers home with their tails tucked between their legs, Birmingham marched to their inaugural victory over Mowbray, who only needed to wait another four days for his second at the Blues helm, masterminding a 2-1 Championship success over Stoke City thanks to Stansfield's strike and Juninho Bacuna's exquisite free kick.
Establishing an eight-point lead over the relegation zone and stringing together a four-game unbeaten run in all tournaments, Birmingham have adjusted to life with the well-versed Mowbray at the helm and now bid to reach the FA Cup fifth round for just the second time in 12 years.
On that occasion, Birmingham were coincidentally denied a place in the quarter-finals by Leicester in the 2019-20 season, and in their first meeting since that 1-0 win for the Foxes, the Championship leaders ran out 3-2 victors at St Andrew's in December's second-tier showdown.
Leicester City FA Cup form:
W
Leicester City form (all competitions):
D
W
W
W
L
D
Birmingham City FA Cup form:
D
W
Birmingham City form (all competitions):
D
L
D
D
W
W
Team News
Alongside the recalled Casadei, Leicester remain without the services of Australia's Harry Souttar and Nigeria's Kelechi Iheanacho, both of whom will soon be competing in the knockout rounds of the Asian Cup and Africa Cup of Nations respectively.
However, Patson Daka's Zambia have now been eliminated from the latter tournament, and the striker will return to the King Power Stadium imminently, although an appearance on Saturday is still unlikely. Abdul Fatawu (suspended) and Wilfred Ndidi (leg) will also miss the game.
Maresca was not averse to making changes in abundance for the win over Millwall and should rotate his resources once again, most likely starting with a demotion to the bench for Hamza Choudhury after the midfielder was charged with drink-driving.
Since battling past Hull in their third-round replay, Birmingham have added Queens Park Rangers midfielder Andre Dozzell to their ranks on a short-term loan deal, but the 24-year-old cannot play on Saturday on account of being cup-tied.
On a positive note, there is optimism that Marc Roberts (calf) and Ethan Laird (groin) will be back in contention for the trip to Leicester, but Alfie Chang (knee), George Hall (hamstring) and Lukas Jutkiewicz (calf) are still in the treatment room.
After ceding his place in goal to Neil Etheridge for the initial encounter with Hull, 37-year-old John Ruddy was back in between the sticks for the replay, and a midfield recall should also be in order for Gary Gardner at the King Power.
Leicester City possible starting lineup:
Stolarczyk; Pereira, Doyle, Coady, Justin; Praet, Winks; Albrighton, Dewsbury-Hall, Akgun; Vardy
Birmingham City possible starting lineup:
Ruddy; Drameh, Bielik, Sanderson, Buchanan; Gardner, James; Miyoshi, Bacuna, Dembele; Stansfield
We say: Leicester City 2-1 Birmingham City
Potentially for the first time this season, the mood in the Birmingham camp may be brighter than their Premier League-bound hosts if recent results are anything to go by, and Blues gave Leicester a proper run for their money a few weeks ago.