Which brand of probiotic to give 4 months old

Hi mummies that give their baby probiotic, at which month did you start and which brand ? Did you see any improvement ? My 4 months old baby has been having watery/runny stools once a day or max 3 times a day. Altho the paediatrician says its normal and the colour is normal, he recommended to give probiotic. He said any brand will do. However theres quite a few brands out there but im not sure which are the popular ones and effective.