My battery dropped 30% in 45 minutes (absurd) on first use, so I performed a factory reset and everything seemed to be going fine.... but now the poor battery problems are back and i'm getting just over 3 hours use (no YouTube, no gaming, just social media).
I've heard if you let it calibrate by letting it drop to 0% a couple times it should settle, but i'm not experiencing this.
Does sound like a dud unit. Keep trying that calibration.
Also check the % left is actually realistic - i.e. does the power actually run out after 4 hours?
You should also be able to see which apps are using the power in the settings.
If the issues continue, it may be worth having a technician access the device. You can drop it off at an Optus store, but it will need to be sent away for a few days. If you live near a Samsung store, they may have technicians on site that can check it out while you wait.