Not actual legal advice. But a friend of mine had a similar problem and this is how he proceeded.
First, do not pay the bill. That's admitting you are in the wrong, which validates the mark on your credit score by the fact that you haven't paid a bill that was due for 4 years.
Second, whether you have a paper contract or not, you entered an oral contract with them to modify your current contract and add a feature to your phone. Hopefully you've at least written down the date and time you talked to the agent, and even better would be to have his or her name.
Third, at this point they've probably refered the charges to a collection agency. I would write the company and the collections agency, and make sure they know you dispute the charges and why you dispute them. I'm not sure how it works in the U.S. but I wouldn't be surprised if there is a mechanism that allows you to dispute an item on your credit score through some sort of arbitration.
Fourth, next time you go the the bank to get any type of loan or mortgage, explain the situation to the agent and tell them you dispute the charges. There is a good chance they will be willing to put a note on your file saying your dispute the charges. In Canada, I know disputes about cell phone bills are common, and if you have a high income and no other credit blemishes, I've heard they generally just ignore entries from cell phone companies.