A crowd of over 84,000 has squeezed into the MCG to see their Ashes-conquering heroes who've already regained the urn after winning the first three Tests of the five-match series.
England opener Michael Carberry has hit an unbeaten 26 off 83 balls and Joe Root is on 11.
Australia missed their chance for an early breakthrough when Carberry was dropped on two at third slip by Steve Smith off the bowling of Ryan Harris.
England captain Alastair Cook battled for 70 minutes for his 27 and shared an opening stand of 48 with Carberry.
Cook appeared caught in two minds and played at a wide ball from Peter Siddle, which the left-hander edged to rival skipper Michael Clarke at second slip.
After averaging 127.66 on his previous Ashes tour, Cook has produced 181 runs at 25.85 this summer and is struggling with his batting form and his captaincy.
Clarke's decision to bowl was the first time Australia had inserted England since Ricky Ponting's ill-fated move at Edgbaston in 2005.
"I can't believe I'm saying this. We're going to have a bowl first," Clarke said at the toss.
"There's enough moisture and grass on the wicket.
"I think what's made my mind up is more the overhead conditions."
A top of 29 degrees is forecast with one or two clearing showers.
Carberry shouldered arms to a ball from Harris in the last over before lunch and was given not out before Clarke called for a review, which showed the ball was just clipping the bails.
Australia named an unchanged side.
England recalled Monty Panesar for retired spinner Graeme Swann and wicketkeeper Matt Prior was dumped for Jonny Bairstow.
Bairstow, 24, plays his 13th Test while it's Panesar's 50th match.
Australia pair Shane Watson and home-town hero Siddle will also play their 50th Tests.