To help pump more try pumping while nursing. Hard at first but works awesome
Also I would take 4-6 oz which is a little high but whoever is feeding him will think he needs more than 3 oz and chances are hes going to be fussy!
You may need to pump to releive pressure but I doubt it. 4 hours isn't that long. if need you can just hand express a little. Just make sure whoever is feeding him to not feed him right before you are going to get him so you can nurse him right away.
Has he started solids? Does he take bottles well?