This is common for 4 month olds-- between 3-4 months, the short napping of 45 mins. tends to appear (sometimes sooner).
First, take a look at her behaviour, does the 45 min. seem to be affecting her?
A lot of times, a change in routine is needed, looking at lengthening the awake time (gradually), helping her fall asleep independently.
By 6 months of age, most 45 min. nappers have extended their naps It takes time and maturity.