No food until 6 months
I think that 4 months old is too young for real food. All the studies show that a baby's gut is not developed enough for real food until 6 months or later. The American Academy of Pediatrics says babies should be exclusively fed breastmilk for the first 6 months. Every other health organization such as WHO and UNICEF all agree solids should be started 6 months or later.
When solids are starts sweet potatoes are a perfect first food, followed by avocados and bananas. Those three rarely cause an allergic reaction, are full of vitamins and healthy fats.