My son had a bit of flat spotting at that age too, and honestly the main thing is to vary his head position during the day and tummy time really helps. Since he's still young at 4 months, there's usually good potential for it to round out on its own as he gets stronger and starts moving around more. That said, might be worth bringing it up at your next PD visit just to check if anything needs monitoring, especially if it's quite pronounced or if one side is significantly flatter than the other.