What flow teat do you have her on - have you tried the fastest teat available?
The amount of formula she drinks really is fine providing she is meeting milestones, gaining weight and has plenty of wet/ dirty nappies. My DD was 8lb 11oz born and stuck to the 91st centile like glue on a max of 25oz a day.
It sounds like she is snacking really, she's taking feeds that are too small to get her to 4 hours but she not properly hungry enough to take a full feed. Is there any chance that she has reflux? If it's not snacking then it could be reflux as that often causes babies to take smaller feeds more frequently.
You can break a snacking habit by stretching her feeds. You need to start one morning, give her her first bottle and then make her wait for as long as possible until you feed her again. You don't want her to get too distressed but you need to distract her long enough to be really hungry. She should take a bigger feed and then next time she waits for at least as long as between the first bottles, or a bit longer if possible. It may help to set a 3 hour 15 gap for a couple of days and then extend that to 3 hours 30 for a couple of days and so on.
What do you think?
Laura