An infant doesn't need any nutrition other than breast milk or formula for the first 6 months. The fat content of these liquids is 50% - no other food item comes close to providing that, and the fat is necessary for energy and for baby's developing brain.
As your infant nears 4 months, he becomes more social, so he is more likely to wake up more often or to try staying awake longer in order to spend more time with you. However, according to what I have learned in "Healthy Sleep Habits, Happy Child" (Marc Weissbluth) and "Baby 411" (Denise Fields), an infant is capable of going without eating for 6 hours straight at 4 months, 9 hours at 5 months, and 12 hours at 6 months. They will wake up more frequently during the night if "trained" to - i.e., if you respond every time, and if you are playful with them.
Sleep is controlled by signals from the brain and has no relation to hunger signals, according to Weissbluth. His study included observations of infants with feeding tubes who were constantly receiving food. Adding cereals to the formula or jarred foods will take up space that formula would, and will provide fewer calories to your baby. Furthermore, infants are not really able to eat solid foods until closer to six months, because their digestive tracts haven't developed enough, and they need to develop better control of head and facial muscles.
I highly recommend the two above books - they have been very helpful with my two boys, age 23 months and 5 months.