Heung-min Son scored twice, with Cristian Romero and Yves Bissouma also netting.
Postecoglou later said: "Football was good and we played well.
"We played well last week and we dominated the game, today was a bit different. The lads had a strong mindset and we got a really good goal early on and we didn't let off after that."
On defence and Micky van de Ven, he said: "There aren't many centre-halves like Micky, he is a great athlete. Last year when we had our back four fit, it made a big difference to us.
"We have to share the workload, at the moment we don't have midweek games, in the international break we have to manage him, Romero and Destiny Udogie as they have had an interrupted pre-season."
On hopes this season, Postecoglou concluded: "I always have hope and I am a optimistic at heart and you need to be in our game, there is plenty that can bring you down.
"Last weeks performance was really good, but when you don't get the result there is always a bit of doubt there."