Many people suggest starting 4 month olds on rice cereal mixed with breast milk or formula. The reason I have heard for this is to make the baby sleep longer at night. A number of sources I regularly consult have said this is unlikely or false.
I have found many sources saying that the gastrointestinal and immunological benefits of waiting to introduce anything besides breastmilk are great. KellyMom.com, a breastfeeding information website run by an International Board Certified Lactation Consultant (IBCLC), lays out many of the same information on this page that I first found in the book Breastfeeding Made Simple.
It turns out that giving a baby something besides breastmilk changes the bacteria in his or her gut to resemble an adult's gut, but a baby's body is not ready for this change until 6 months at the earliest. It can lead to a weaker immune system, food allergies, and tummy aches. KellyMom.com also says that if "adult food" is started too early, energy that should be used for growth gets diverted to the immune system to prevent infections and diseases due to the lack of protection from breastmilk.
One thing most people don't realize is: formula may as well be "adult food".
It is made from cow's milk (or soy) mixed with vitamins and minerals and is not natural "baby food". Check the label on a can of formula in the grocery store. The first ingredient is whey protein. Then go to the aisle with the protein bars to find a can of the stuff sold for adult consumption. I used to put some in fruit smoothies when I was pregnant to help me hit my 50-75 grams of protein every day. You will also find whey protein as a main ingredient in those protein bars too.
This means that it is not a good idea to supplement your own milk with formula part of the time. You are sacrificing some of the benefits of the breastmilk. Of course, sometimes it may be difficult to give your own milk to your baby all of the time, but feed your baby formula only in an emergency. I held on to that can of formula given to me at the hospital "just in case," but I have since gotten rid of it because I decided that I am setting a goal to never give any of my children formula. (It's an atrocious practice for the hospital to give out formula anyway! More on this issue later, though.)
Investing in a good double-pump breastpump will make pumping milk easier and thus more likely that your baby will only get your milk when you can't be around. I use the Medela Freestyle pump. It's pricey, but totally worth it! I can pump a whole bottle (5 ounces) in about 5 to 6 minutes by double pumping, that is pumping both breasts at the same time. My husband calls it the "boob-a-tron" because I look rather silly all hooked up to it, but it's really fast.
Don't even bother trying to pump one breast at a time. It doesn't work very well. Don't waste your $150 on the single pump, just splurge the $300+ on the really nice hands-free double pump. It's for your baby's health.