at four months of age not every child achieves neck holding and this may be the case with your child. as you said your child tries to keep her head in line with trunk though shortly, you should wait for few weeks more if all other milestones are within normal limits.
nonetheless if you really think that the neck is too weak and the child is also late to other children in other milestones like social smile, recognizing mother, fixation of gaze etc, you should contact a pediatrician
for proper evaluation.
one more thing i would like to know is about the posture and body tone of the baby, if the tone is more or less than normal or there frequent backward arching of the body it may be a sigh of Cerebral Palsy
. but all these thing need a consultation with a pediatrician.
treatment of Cerebral Palsy is multi pronged and it needs support from different fields like your pediatrician, orthopedician, physiotherapist etc, CP is not curable but the child can be trained to lead a normal life under professional guidance