We've been to Verona a few times, but unfortunately have never watched an opera in the arena. However, I have heard (on another forum) that for your comfort you should take some kind of cushion to sit on, as the stone seats get a bit uncomfy. Apparently you have to get to the arena at least an hour before the performance starts, so your 4 hours turns out to be a lot longer. Also, it can get chilly in the evening, so take a jumper just in case.
There are plenty of cafes and bars around that area, so I would nip in to one of those for your comfort break before you enter the arena.
I had a look on their website, and although they have a map of the seating areas, they don't show if there are any toilets unfortunately.
Verona is a lovely city, and I'm sure you'll have a wonderful experience.

We went to the arena during the day last Sept. There are toilets there, but not many of them and not very nice if memory serves me correctly. There are some public toilets in the square but I'm not sure if they would be open at night. But there are loads of bars around, as Tishtosh says.