4 month old has mild flat spot on back of head. how do i correct this? will it even out when she sits up more?

Very common: Its common to get flat head at the back due to prolong laying down flat on the back. While awake and you near , give her tummy time , lay her on her tummy , it will help .

Flat head: Since babies spend less time on their stomach more and more have some flattening of the occiput (back of skull). If it is mild the only intervention is trying to keep the baby off that spot and more of their side. The skull is very moldable at this age and is using easily corrected. In severe cases the child can be fit for a helmet which keeps pressure off the occiput. Plagiocephaly is the term.

Plagiocephaly: it is not uncommon for babies to develop flat spots on their heads early on. Make sure your baby gets "tummy time" every day. That takes the pressure of those areas and also encourages development of neck and torso strength - both of which are vital for being able to sit unsupported. Most times these flat spots go away once the baby spends less time flat on their back.