Introducing cereal in your babies diet is good at this point. It also helps them sleep better! Then you gradually add fruits and veggies.
Four months is the perfect time to start feeding solid foods to your baby. The baby needs more than just a liquid diet. You can start buy giving rice cereals along with some milk mixed in it or water. The baby might get hungrier at this age and may wake up more frequently at nights. The baby may get cranky very soon and you may have to feed him at regular intervals. The baby gains maximum proteins and minerals at this age. There are many baby foods available in the market. You can also start with fruits such as oranges which is easy to chew.