If I have four felony convictions, how much would it cost to get records sealed?

You can't get the felony convictions sealed. There is something called "expungement" of criminal convictions, so perhaps that's what you're referring to. In Missouri, only a few types of felony convictions are eligible for expungement, and only after 20 years of good behavior. You need to consult a criminal defense attorney in the city or county where the convictions were entered and find out if your felonies are on the list of felonies that qualify for expungement.