Definitely talk to a doctor, but everything I've read (and everything my ds's pediatrician has told me) says that 4 weeks is WAY too early for rice cereal. Even formula companies say don't give it to them before 4 months old - they may develop problems later on like gluten allergy or, worse, diabetes.
It's very normal for a little one to be eating every 1.5 hours at that age. They really need it... it means they are growing. It's a good thing! You need to 'fatten them up' because this will help their nervous system tremendously!
I hope your doctor is knowledgeable about breastfeeding, too, because some know more about formula feeding and give advice based on that. There is also so great advice and very knowledgeable people in the Breastfeeding forum. Be sure to check out the information there.
My personal experience: my bm was getting less and less, and my ds was still really hungry, so I started giving him some formula to 'top him off'. I mention this to his pediatrician who told me that this will only make my milk supply even less! The key is to drink as much water as possible, make sure to eat at least 3 meals a day (preferrably 5 smaller meals) and get LOTS of rest!!
Let us know how things are going.