My baby is 4 months. Can she start using a walker/bouncer? She has good head control and can sit supported.

When able to sit up. Walkers and jumpers do not help a baby learn to walk. Walkers and jumpers are ok to use under adult supervision, and for less than 30-60 minutes at one time. They are not for keeping a toddler in, without supervision. Dangers include crashing into furniture corners, flipping over, falling down stairs, and sliding through the seat (possibly getting tangled up and possibly getting choked).