It was a real goal-fest at the King Power Stadium.
Leicester dominated the second-half after Vardy's penalty and continued to put QPR on the back foot.
The Foxes get a big win which should give them some confidence as they turn attentions back to the league campaign.
The hosts have dominated the game but struggled to break the deadlock.
When Nkunku's penalty was saved, heads seemed to drop.
However, a wild deflection from a Tosin strike saw the ball end up in the back of the net to open the scoring in West London.
It was comfortable in the end for Leicester as they put their Premier League struggles to one side to go goal crazy.
Jamie Vardy celebrated his 38th birthday with a strike from the penalty spot to ease the Foxes' nerves after a first half that produced five goals.
QPR impressed before the break and gave Leicester a few scares.
But Ruud van Nistelrooy's cut loose after the break with James Justin scoring twice.
Leicester saved the best until last as defender Wout Faes unleashed a spectacular 30 yard drive for the Foxes' sixth of the day in the King Power Stadium fog.
A downward header from Ryan Yates five minutes before the break has put Forest on course for a place in round four – and a seventh consecutive win.
Yates also rattled the crossbar for a second-string Reds side – with Nuno making 11 changes ahead of a huge Premier League clash against Liverpool on Tuesday.
Isaiah Jones had a great chance to score a debut goal for managerless Luton, with Carlos Miguel denying him from close range.
90+3. It is a real drubbing at the King Power Stadium.
The Foxes have really turned the knife in this second-half.
Faes adds an injury-time sixth goal to put the icing on the cake.
A dominant turnaround from Iraola's side.
They trailed by a goal before Kluivert and a brace from Ouattara turns the game on its head.
Bournemouth look dangerous whenever they get into the visitors' half at the moment.
CHELSEA are winning but Morecambe are carrying the fight.
And keeper Harry Burgoyne embodies their spirit with a penalty save and great reactions to keep out another ferocious shot.
Sad for the romantics that he has been breached by a hugely deflected shit from Tosin which has put the Premier League aide ahead.
Dango Ouattara scored twice before the break to give Bournemouth a 3-1 advantage.
Kluivert slipped a neat 34th minute pass through to Ouattara to fired through goalkeeper Wildsmith’s legs.
And ten minutes later captain Brooks finished off a swift counter attack by setting up Ouattara for his second
New Plymouth boss Miron Muslic will be impressed with what he has seen from his new side who have more than held their own against Brentford in the first 45 minutes.
Muslic, who is sitting in the stands at the GTech Stadium. will officially take control of the team on Monday following his appointment yesterday.
Despite sitting bottom of the Championship and with the leakiest defence in the second tier, the Pilgrims have looked comfortable against the Bees and a threat going forward with Morgan Whittaker and Callum Wright impressing for the visitors.
Plymouth keeper Conor Hazard has made two routine saves to deny Fabio Carvalho but the Bees, without top scorers Bryan Mbeumo and Yoane Wissa, who are on the bench, look rather toothless in attack.
Thomas Frank will not want this match going to a replay with Manchester CIty the visitors here in midweek, but he may need to turn to his top scorers in the second half.
Two goals from Rutter send Hurzeler's side in at the break in a position of strength.
They have looked like scoring every time they come forward.
It looks like they have too much attacking threat to not win this game from the position they find themselves in.
40. Mitchell gets his second goal of the game as he strikes a shot from the centre of the box into the bottom corner of the goal.
This is end-to-end stuff and you could still not call a winner.
The ground is bouncing now the hosts lead.
Giant defender Caleb Taylor gave the Baggies a shock but deserved lead against their Premier League hosts.
Grady Diangana headed John Swift’s 14th minute free-kick onto the bar and Taylor nodded home.
This livened up the Cherries and Max Aaron and Kustin Kluivert shot just wide.
But top scorer Kluivert equalised on 27 minutes when he smashed David Brooks’ cross high onto the Baggies net.
44. This may well be game over for the visitors.
Even when they try to get forward themselves, Bournemouth pick them off on the break with their pace.
As they steam forward, is it played to Ouattara who passes into the back of the net to give The Cherries a two-nil lead.
39. It is a lucky goal to give the hosts the lead.
Tosin looks to get a shot off but it takes a massive deflection and wrong-foots the Morecambe keeper.
Not that Chelsea will be concerned, they felt they should have got to take that penalty again.
40. Finally Forest break the deadlock at the City Ground.
They have been dominating this game but a header from Yates into the bottom corner to open the scoring.
Up until that point Luton had done well to hold the hosts at bay.
37. Brighton take the lead as Minteh lays it back for Veltman to cross into the box.
Rutter is there to take advantage with a neat finish.
Great start for The Seagulls in this tricky fixture.
And there it is, Jamie Vardy marks his 38th birthday with a goal as Leicester take control of the tie.
The Foxes skipper scores from the penalty spot to make it 4-2 after handball from Harrison Ashby;
Vardy's goal gives Leicester some breathing space and who's to say he won't sign another deal next summer?
He might have won a second penalty after going down under a Kenneth Paal challenge, but ref David Webb waved play on.
Leicester are tightening their grip after an impressive first half, and James Justin has fired a second goal of the day into the far corner.
We still have thick fog here, but Ruud van Nistelrooy's side can see a clear path to the fourth round.
Not much happening at Carrow Road although Amankwah Forson did rattle the post with a superb left-footed shot.
The Ghanaian is only 22 years old but looks like a decent player. He arrived in English football last summer.
I reckon he will be in the Premier League next season, and maybe not necessarily in Norwich colours.