It's actually different for every baby.
During the four month age though you'll usually get a lot of 45 min naps. That's normal development. It will pass, but day sleep has to work itself out, which usually happens somewhere late in the 4th month or early in the 5th.
During that time you'll want to start to move to a 2 hour waking time. It may take a few weeks, but it will happen. You can also start looking to the clock instead of wake time for naps. They should happen after around 2 hours of wake time, but you might think about setting up a nap schedule with a bit of flexibility (a half hour to an hour around a nap time). This means you start getting him/her up the same time every morning (or around the same time), and put them down for naps at around the same time every day, with the flexibility fitting in when a nap is cut short. This applies to most days, not all days, because obviously you have a life too
They should be getting 11-12 hours of sleep at night, and 1 nap of 1.5 hours, and another nap of 1.5-2 hours. The third nap (if necessary) is a cat nap with a max of 45 mins. Obviously this won't happen every day. In fact, it's lucky if you get this once a week. I would also put him/her to bed about 11-12 hours before they normally get up in the am. Biologically, the best time for naps is mid-morning, early afternoon and early evening.
Hope this info helps!
*HUGS*
Sonya =P
