I would seriously consider joining Qantas Club (or the equivalent) so that you can use the lounge during the 4 hours.

There are lots of seats in an airport, so you will have plenty of places to sit down. You font have to spend four hours on your feet

I did this with a baby and then a toddler plus baby. I found a quiet corner of the airport (they do exist!) and we spent most of our time there. Blanket on the floor, snuggles on the lap. We got through it, so will you

A bottle of milk/breast or powder, or a bottle of water is also good. In the plane you should have the bassinet in the front row if it’s a long haul flight which is a great relief. When you get to your stop over as the others have suggested a blanket on the floor somewhere near a comfy seat. Or as one other said check with the airport or your airline prior to leaving if they have a pram that can be hired/borrowed whilst in the airport.

If you can breast feed of bottle feed for take off and landing thats the best as it will help pop the ears and is a great distraction.

Take a rug and put bub on the floor in a nice corner while you have a coffee somewhere. Also ask if the airport may have a pram you can lend, some may have this option.

We just did this with our six month old. I just has a couple of small toys, rusk and dummy to occupy her on a blanket on the ground so she could roll around etc. hubby and I split tasks so I took bubs in the ergo whilst he dealt with the passports (we’re flying to NZ via Sydney) and laptop etc. BReast feeding really helped as just fed when she needed it but again on take off and landing. She was amazing but you’ll also find other people will help keep them entertained by pulling faces etc around the airport and on the plane