It's really however much he wants. All babies are different. I would probably assume the amount you are expressing and giving him in breast milk would be a rough guide, though you're right that 4 oz per feed at that age wouldn't usually be very much. If he's feeding from the breast at other times, he's likely then taking much more and making up for it later. If you're exclusively expressing, then I would probably assume it would be the same. Breast milk is easier to digest and often less dense compared to formula, so usually babies take more breast milk relative to formula, especially to start as formula doesn't taste as nice. I would probably aim to leave 5-6 oz per feed and see how he does with that (and also leave more for whoever is caring for him while you're away, so they can make him up another bottle if it's not enough). You want him ideally to not finish every bottle, leaving some about half the time. If he's finishing 4 oz. every single time, I would offer more until he starts to leave some after some feeds, but not every feed. That's when you'll have a good idea of how much he usually wants. But it's still very variable. Sometimes they'll take 3 oz, sometimes 8 oz, but there is usually some pattern to it after a while, which you'll be able to work out so you can better predict how much he'll want. At 8 months, my daughter was having 7-9 oz 6 times a day (5 during the day and 1 feed overnight) plus 3 meals a day, but they are all different.
