England are 1-0 up with two to play against Australia heading into the start of tomorrow's clash here at Headingley.
Flintoff, who has said this will be his final Test series, led England to glory with the ball in their 115-run second Test win at Lord's and then top-scored with the bat in the drawn third Test at Edgbaston.
But all the while he has been struggling with a right knee injury which has so far required several pain-killing injections during this series.
He batted during practice here on Wednesday but did not bowl.
The party line from England has been that Flintoff won't play unless he is "fully fit" but the term can mean different things to different people and it would be a big decision for the likes of the England selectors, England captain Andrew Strauss and coach Andy Flower to leave Flintoff out.
"We're expecting him to play because that's what he has done over the last three Tests," said Australia all-rounder Shane Watson.
"I'm definitely preparing for him to be fit and playing. He's a tough guy. And for him to be able to do what he has done has been awesome.
"For me as a batsman to be able to face Freddie fired up and going hard is one of the biggest and best challenges in my career.
"Let's hope he's fit and firing and continue the challenge."
England have certainly given themselves plenty of options if Flintoff is ruled out.
Uncapped South Africa born batsman Jonathan Trott is in a 14-man squad as are left-arm quick Ryan Sidebottom, who began his career at Headingley with Yorkshire, and fast bowler Stephen Harmison.
"If Fred (Flintoff) can't play, Trott is an option to bat at No 6 then we balance the bowling attack thereafter," said Flower.
"We trust our top six to score the bulk of the runs and the others have the onus of taking the 20 wickets.
"If someone had offered us being one up with two to play at the start we would have taken it."
But while England's quicks have been a threat when the ball is swinging, they have a much reduced force when overhead conditions have not been helpful.
Australia, who have not been behind this far into an Ashes series since England's victorious tour 'Down Under' in 1986/87, have issues of their own.
They have yet to take 20 wickets in a match this series although whether they will recall experienced fast bowler Brett Lee, who has not played in the first three Tests of this series because of a side injury, remains to be seen.
In-form batsman Michael Clarke has an abdominal problem, although he is expected to be fit after scoring hundreds in the last two Tests.
Meanwhile, wicket-keeper Brad Haddin hopes to be back despite missing the third Test with a broken finger.
Both Peter Siddle and Mitchell Johnson have proved expensive with the ball, as was Watson at Edgbaston, but Australia may decide to do without off-spinner Nathan Hauritz and play an extra quick.
Ricky Ponting is desperate not to become the first Australia captain in more than a century to be a losing skipper on two tours of England and Watson said, "The intensity's starting to rise up because we need to win."
This match will be played out against a backdrop of a building site at the Kirkstall Lane End of the ground where work on the new pavilion will be delayed during the Test.
Of greater concern to Australia, after an Edgbaston Test where effectively two days were lost to bad weather, will be the ability of the outfield to cope with any rain.
In May, despite the installation of a new USD 940,273 drainage system, a one-day international between England and the West Indies at Headingley was washed out without a ball being bowled.
But Yorkshire chief executive Stewart Regan insisted, "We have since seen an 85 per cent improvement in the infiltration rate of the water flowing through the ground."
