This is a question that stumps a lot of parents, because the goal of the first four months is exactly the oppositeof what comes next. In the first three to four months, we wrap and swaddle babies because they have immature nervous systems that need containing—an unswaddled baby sleeping on her back (where we put her for safety reasons) will naturally wake a lot because her arms and legs go flying periodically.
When babies develop new motor skills, like the ability to roll to their sides or tummies, they're ready to be set free. The goal at this point is to give them as much freedom of movement as possible. When babies get comfortable rolling and moving in their cribs, they can find their own favorite sleeping positions—that's the ultimate goal for sleep.
So the best night attire is fitted cotton jammies (or an over layer of a blanket sleeper, which is another footie pajama but for cold winter nights). This is preferable to a sleep sack, because it allows the baby to freely travel around the crib. We want babies to "own" their sleeping space, adjusting and moving as they see fit.