A baby around the age of 4 months may begin to outgrow his or her swaddle. What can happen is that their arms become loose and they may begin to flail them. Many tired moms have tried to re-swaddle their baby at night only to find he or she becomes loose again.
This new pattern can be hard on both the baby and the mom since their night time sleep is being interrupted. My suggestion is for the baby to get lots of “tummy time” during the day so s(he) can flail away and get tired in the process. Babies learn quickly and it won’t take long for your baby to be “sleeping like a baby” once again!