A 4.5 month old baby with phlegm should keep warm and avoid getting cold at this time. You can pat your baby’s back properly more often every day to promote the vibration of phlegm discharge. Continue breastfeeding because breast milk contains a lot of water, which can dilute the phlegm and make it easy to expel. Moreover, it contains a lot of rich immunoglobulins and natural antibacterial factors that can resist pathogens. Nebulized inhalation treatment with budesonide nebulizer solution can be used if necessary. If the sputum is very abundant and not easily expelled, this is the time to choose a suction machine to suck out the sputum and prevent the increase of inflammation. If necessary, blood tests, x-rays, etc. are needed. Even sputum can be taken for culture of pathogens, drug sensitivity test, etc., to clarify the cause of the disease and carry out further treatment.