My 4 month old is doing this too - I combi feed but only give top up bottles in the evening and BF the night and the rest of the daytime.
She seems to only have a decent feed when she is dozy or really ravenously hungry, like first thing in the morning! Any other feed she is on and off, distracted easily and often drinks, then comes off and spend a minute looking around before going back on for a couple of sucks then looking around again.
I also time feeds and have found that she has dropped feeding time too.
I am posting to reassure you more than anything as I am not worried at all... She is still doing the approximate same duration of feeding every day, is alert and has plenty of wet and dirty nappies. I think the feeding time has reduced as she has got better at feeding - she used to be on there for 40 minutes as a newborn, then it reduced to 20, now it's 10 - 15.
I am putting it down to a developmental thing... I think she has discovered new things about the world and is really interested in those at the moment. As she is not showing any other signs of anything not being right I am just waiting to see whether it changes, or whether this is the new feeding pattern!
By all means seek help from a lactation consultant or breastfeeding support line etc to check everything is ok - I am sure they will reassure you that it is normal. DD always finishes her bottles too but that's because the flow is faster than from the breast so she get a quick, easy feed and not because she prefers the bottle. My DD gets especially grumpy in the evening... Does your DD get grumpy and fussy at the same time each day?