I went back to work when my daughter was 4 months (she was 5 weeks early). She was on demand for feeding and naps then, and only went to a "schedule" for feeding when she entered a toddler program. As for pumping, it all depends on how responsive your body is to the pump. To keep up with my daughter I had to pump every 2 hours for the first few months back. I also pumped twice (each morning) over the weekend. Milk supply will respond to demand - your body will keep up with her if you are willing to keep nursing and pumping as much as she needs. If not, you can consider supplimenting. I'd rather supplement than try to put a tiny baby on a schedule.
I don't think a feeding schedule would decrease the amount of milk that your daughter would take, since she'd just be more hungry by the time she's fed. Plus, I am really, really worried about "schedules" for preemies, who need all the calories they can get.
Also, just a warning, be prepared for the 4-month sleep regression. It can be a whopper, especially when you work and can't nap with her.