Did you have a contract with this guy? Did he give you a quote on paper? My uncle used to hang siding and he always measured because he said that if you measure correctly, your time/labor costs were always included. he took photos, even back in the 80s of the houses. He never had to go back and ask for more money and he was always paid in full at the end of the project. Only a couple of times did he ever have to put a contractor's lien on a property. So is this guy asking for an additional $4523.00? How much work has been done so far? That's pretty steep. Who did the measurements. Did you get any referrals before hiring him? Start taking photos of his work and tell him you would like to drive by some of his past work.