My 4 month old will only eat 2 onz., unless I sleep her first. What is wrong?

She was eating well(4 OZ every three hours) up until a month ago. Now she will only finish her bottle if I sleep her with her pacifier and then switch it to her bottle. She will happily down her whole bottle this way. I have tried feeding her what she wants(2 OZ) and holding her off until the next feeding so shell be hungry,and in this way get her on a schedule but instead shell cry and stop eating completely, the last time she went 5 hours on 1 OZ, she simply would not eat but was crying like crazy.Im afraid that what began as an attempt to keep her fed has become or will become a very bad habit. What is causing this strange behavior? And What can I do to fix it?