Eh I understand your worry! Honestly 4 months is still quite manageable, a few of my mum friends bawa baby umur 3-5 bulan naik flight, and the main thing is just to prepare yourself mentally and pack the essentials (diapers, wipes, extra clothes, milk stuff). The cabin air won't harm baby, just make sure you have a pacifier or feed during takeoff and landing to help with the ear pressure, and try to book a flight time that works with baby's sleep schedule kalau boleh. Bring a soft carrier or lap seat to keep baby close during the flight, paracetamol kid-safe just in case, and honestly don't stress too much, babies are tougher than we think! Good luck with your trip back to Sabah, I'm sure it'll be fine!