can you post what her general day is like ie when she usually wakes for the day, when her 1st, 2nd, 3rd naps usually are, bedtime... that will help provide more answers/advice/suggestions.
naps at 4mos are still developing so it's quite normal to have crappy naps at that age. some babies are natural sleepers; others are more challenging and 45min naps are quite common until their brains begin to organize day sleep - this generally happens around 5-6mos but all babies are different - some do and some don't.
my suggestions without knowing your baby's general day is don't go beyond 2hrs wake time. if she is showing you drowsy signs before 2hrs, then she is getting tired and start your nap routine the second you see her drowsy signs (activity levels slowing down, glazed eyes, yawing etc). if you begin nap routine when she's fussy and crying or rubbing eyes etc, then she is entering the overtired state and will most likely give you a short nap.
the 3rd nap is usually just a catnap that lasts 30-45mins just to get them to bedtime. if she's not napping well during the day (ie. catnapping all day), put her to bed early for the night.
please post her general schedule so we can add more specifics
HTH