After bundling out Australia for 167, South Africa lost both the openers early for 29.
South Africa in a tricky situation depended on du Plessis and JP Duminy to build a partnership. They put on 56 for the third wicket and with the score on 85, Duminy was dismissed.
Then Faf du Plessis and Rossouw stitched together 55 run partnership and put victory beyond Australia. du Plessis scored a brilliant 69 off 87 balls and was out with the score on 140. There were no hiccups for South Africa as Rossouw and Behardien completed the formalities for South Africa.
South Africa won the match by six wickets and lead the series 4-0.
Earlier, South Africa bowled out Australia for 167, the lowest total of the series, despite resting three of their frontline bowlers in the fourth one-day international against South Africa at St George’s Park on Sunday.
Opening bowler Kyle Abbott bowled both Australian opening batsmen inside the first three overs and finished with four for 40, while unorthodox left-arm wrist spinner Tabraiz Shamsi took three for 36.
Mitchell Marsh and Matthew Wade made half-centuries but the next best batsman was tailender Chris Tremain, who made 23 not out.
South Africa, having clinched the five-match series by winning the first three games, picked what was effectively a back-up bowling attack, with fast bowlers Dale Steyn and Kagiso Rabada both rested along with leading spin bowler Imran Tahir.
They were replaced by Abbott and spinners Shamsi and Aaron Phangiso.
Abbott, who bowled a full length on a pitch which offered some sideways movement, bowled Aaron Finch with the third ball of the match and then bowled David Warner with the first ball of his second over.
George Bailey was leg before wicket to Dwaine Pretorius as Australia slumped to 12 for three.
It became 49 for five when Shamsi deceived Australian captain Steve Smith and Travis Head, trapping them both leg before, in the space of three balls.
Marsh and Wade put on 62 for the sixth wicket, while Wade and Tremain added 46 for the ninth wicket.
Marsh made 50 off 72 balls, while Wade hit 52 off 58 deliveries.