Most precocious puberty is idiopathic, meaning they don't know what is causing it. I had it as a kid, and at four, it is unlikely to be related to any particular enviornmental cause. Things like hormones in meat and milk, etc, are certainly likely culprits in the general lowering of puberty age, at such a young age it is unlikely to be related.
They will do an xray of her hands to see what her approximate "bone age" is. The hands work well for this test because the older you are, the fewer bones you have. It helps determine how she is growing. They will do blood tests for various hormones.
However, I would push very hard for a referral to a pediatric endocrinologist. They should do a CT scan or something of the type to rule out issues in the brain that might be causing precocious puberty, but most cases are not related to that. If it is determined that she is entering puberty, there are medications that she can be given to stop this. The reason for doing this is that going through puberty early stunts growth. (It sounds ironic, bc they are usually the tallest kids, but they finish all their growth early and don't grow anymore.) Also, the emotional and social and developmental issues that early puberty encompasses are pretty all encompassing, more so than one might think. It can be scary to give your kids medications like this, but in this kind of case, I would do it for my daughters.