Good grief! I cannot believe all the crazy responses you have gotten. Your baby is too young for fruit and juices, etc. I assume you have only fed her up to this point the cereal (which is fine to start at four months BTW)... you can wait until 6 months as others have suggested but the general guidelines are to start with cereal somewhere between 4-6 months.
I will tell you what our pediatrician told us when this subject came up. Babies at this age can be inconsistent with their BM's. Some days they may poop 7 times a day or they could go 7 days without pooping. Generally, it is not because they are constipated, but more likely that they are hitting a growth spurt. I took my daughter in to the doctor because she had not pooped in 3 days and I was terribly worried. He checked her abdomen to make sure she wasn't distended or that there wasn't any kind of blockage. He told me to call them back if she had still not gone after 5 days. At 5 days, she finally went. She was not constipated. It just looked like any other breast milk poop.
If you are concerned, yes, please do consult your pediatrician, but I am pretty confident they will tell you what I just told you. I am highly confident they would not tell you to introduce new foods or juices that your baby's system is not able to tolerate yet.