Every pregnancy is really different, and some mums do gain less the second time around especially if they're more active or their appetite is different, but since the baby's estimated weight is also on the lower side it's worth flagging this with your gynae or polyclinic just to make sure everything's tracking okay. Better to have them check and put your mind at ease than worry through the rest of your trimester, and they can advise if you need to tweak anything with your diet or nutrition.
while weight gain can vary between pregnancies, gaining only 4kg by 3rd trimester... your gynae can better monitor & offer guidance 🙏💕