Bell found a willing partner in Kevin Pietersen as they put on an unbroken 91-run stand after the loss of openers Andrew Strauss and Alastair Cook after winning the toss here on Tuesday.
England found runs hard to score against persistent Australian bowling on a pitch losing its early life.
At tea, England were 149 for two with Bell unbeaten on 58 and Pietersen on 36.
England are trailing 4-0 and facing the prospect of the first Ashes series whitewash in 86 years.
Bell scampered through for a single off Shane Warne for his fourth half-century in nine innings in the series.
At tea the Warwickshire right-hander, who scored 87 in the second innings of the third Perth Test, had batted for 164 minutes off 120 balls with six fours.
Pietersen, accused by Australian coach John Buchanan before the Test of not being a team player, was frustrating the Australian bowlers as he frequently advanced down the pitch to unsettle their bowling rhythm.
Earlier, Andrew Strauss failed again and was out in the 15th over after being dropped two overs earlier.
Strauss, who has scored only one half-century in nine innings in the series, needlessly chased a ball outside his off-stump and was caught behind by Adam Gilchrist for 29.
Cook fell in the third over after lunch, caught behind off an inside-edge for 20 to give Stuart Clark his 22nd wicket of the series.
Justin Langer had a poor start to his 105th and last Test match when he put down Strauss when he was on 21.
McGrath coaxed an edge which flew low to third slip, but Langer could not hold on to the two-handed chance.
McGrath, playing in his 124th and final Test at his home ground, bowled without luck, beating the bat a dozen times as he got the ball to move off the pitch.
Australian cricket is farewelling Test greats McGrath, Langer and Shane Warne, who have all announced their Test retirements.
McGrath and Warne, who have 1,263 Test wickets between them and have been the scourge of English batsmen for a good dozen years, have been match-winners in a golden era of Australian cricket, while the plucky Langer forms part of Australia's most prolific opening partnership with Matthew Hayden.
The Test got away 70 minutes late after morning rain but play was scheduled for a later finish to accommodate the day's quota of overs.
Ricky Ponting's team has the chance to inflict the ignominy of a 5-0 series clean sweep, last achieved by Warwick Armstrong's 1920-21 Australians at home.