Both of my children had big sleep changes right around 4 months. When they're smaller than that, they sort of have that newborn thing where they'll just drop off anywhere, and for any random amount of time. But around 4 months, both of my kids started needing a nap "schedule,"(Not a clock-run schedule, but a baby-run schedule) in that they needed a nap when they had been awake for about two hours. My son was a flexible sleeper, and I could ususally get him to fall asleep in the sling or the frontpack. But my daughter could only fall asleep in her hammock or, if I was driving somewhere, in the car. If they got overtired, they would fuss while they were falling asleep, refuse to nurse, wiggle all around and need to be swaddled.
That stage of needing many short naps doesn't last very long. By six months or so, they both took a morning nap and an afternoon nap, and that was all it took to keep them happy.