Hi! Don't worry, this is really common at 4 months, and most babies naturally fill it in by around one year as they start moving around more and spending less time on their backs.
The hair-cutting myth is just that, a myth. Cutting or shaving won't speed things up; it's the friction from lying on their back that causes the bald patch, so the real help is tummy time as much as your baby can manage. A friend of mine swore by switching to silk or satin sheets instead of regular cotton ones, and she said it made a noticeable difference. Some mums also use a little coconut oil on the spot, though nothing magic, it's really just time and less pressure on that area.
Since she's still so young, I'd honestly just keep doing tummy time and see how it goes over the next few months. But worth a quick mention to her paediatrician at the next check-up just to make sure there's nothing else going on!
Anonymous