I have gone through your post in which you have mentioned that your 4 month old received a skull fracture
& his CT scan
& X-ray has not revealed any signs internal bleeding
, which has a very favorable prognosis.
Another reason of concern in skull fracture is presence of air inside the cranium (skull).
This entrapped air tends to expand on ascending to high altitude (such as during air travel) & cause severe pain
So it is important to observe the child for 1 week & if there is no improvement or if there is any aggravation of symptoms, it is better to consult a Neurologist
But there is nothing to worry as these symptoms usually subside within a week & I hope he will be perfectly alright by time of the trip.
Wishing your child a speedy recovery.