Don't panic yet--seriously, stress is one huge contributor to decreased milk supply.
Don't start cereal yet. Reason 1: 4 months is too early to start solids and baby cereal is one of the worst foods to start with--the AAP recommends starting at six months, because 6 months is the minimal age where the baby's gut is mature enough to possibly handle solid foods. Before then it just goes straight through and can cause damage.
Reason 2: If you want to increase your supply, making your son sleep longer and spend less time breast feeding won't do it--in fact it will probably lead to a decreased supply even if you pump during the period he is asleep. The more you breast feed the more supply you will have. Pumping will not stimulate production the same way direct feeding will, so breast feed your son as much as you can while you are at home with him.
Take in other factors for your son's changes in nursing patterns. He be nursing more for comfort since the whole mom not being right there for him 24-7 is still relatively new and he needs the reassurance that you are back after his grueling day at work (ie. the babysitter's) ;-). 4 months is also a common period for normalization of milk production, so whereas up until then many people have an abundance of milk, around 4 months there doesn't seem to be extra, so it feels like there isn't enough. It's also a common period for a growth spurt to start, so the baby wants to nurse constantly.
If you can pump while at work, then that would be the best time to try to increase your supply with the pump. If you can't then nurse as much as you can as soon as you get home, during the evening and night and in the morning before you go to work. It's called reverse cycling-- Kelly mom: http://www.kellymom.com/bf/normal/reverse-cycling.html
On the same website (Kelly mom) there's information on foods and herbs you can consume to increase your supply (and if I remember correctly they also have a list of stuff to avoid if you are having supply difficulties)
If you are still worried about your supply dropping, call your local LLL and see if they can give you some advice or get you in contact with other local working moms who can help you through this period of adjustment.