Fungal infection

Oh poor little one, 4 months is such a chunky stage and those folds trap so much heat and moisture, totally common for fungal infections to crop up there. From what I've heard, the key is really keeping those areas as dry and aired out as possible, so after bath time make sure you gently pat completely dry in every fold before dressing baby, and if you can, let the skin breathe a little before putting clothes on. Someone in my mum group mentioned that their PD prescribed a mild antifungal cream (like ketoconazole) and it cleared up within a week or two, though a couple of mums said they were also given a short course of low-strength steroid cream to calm the redness down first, so it does seem like doctors take slightly different approaches depending on severity. The tricky part is prevention once it clears, since Singapore humidity means the folds get sweaty again so fast, so staying on top of drying thoroughly really does seem to be the long-term answer. That said, since every baby's skin is different and some creams aren't suitable for very young babies, it's really worth going back to your doctor or a paediatric dermatologist to confirm the right combination for your 4-month-old specifically.