Ideally, babies should start solids only when they are 6 months old, not 4 months. Reason being, their gut is still immature when they are below 6 months old so they are more prone to getting allergy, indigestion, infection and other illness. Protection of breast milk also decreases once you stop giving total breast milk. Babies' nutrition comes from breast milk below 1 year old, so let's not be too eager to start solids. You can read here for more info on why we should delay solids: kellymom.com :: Why Delay Solids?
I attended workshop on babies weaning. According the PD, we can start giving semi-solid food once the baby is ready to wean. Not necessery to start on the 6 mths.At the workshop, there was a baby started his semi-solid when he was not even 4 mth. The PD said is ok as long as the baby's body can accept the food. But the first food to intro to baby must be brown rice cereal. Feed the baby for 4 days ( 1 tablespoon with 20ml of milk) to observe if he is allergy to it. If he fine with the cereal then we can increase the portion.When they are 6 mth, we can feed them with porridge mixed with fish or veg.