Ok - she could be a bit constipated by the change to formula, it could be because her system needs a few days to adjust, formula is much harder to digest than breast milk. You can offer her a little cooled boiled water between feeds if she seems unsettled but no more than about 2oz over the day.
The size of her feeds, 3-4oz is a fairly normal feed size for a 6 week old, some are taking more at this age but not that much. I would stick to offering her 4oz bottles every 3 hours for now, unless she is giving you hunger signs earlier. The fact that she is having plenty of wet nappies and is content to go 3 hours between feeds is a good indication that she is getting what she needs.
Keep an eye on the poo situation, although not every baby goes every day so it isn't always a problem if they don't. If she really seems to be struggling then it would be worth speaking to your Dr or Health Visitor for advice.
HTH!
Laura