My favorite booked car service charges 3,200 baht for a sedan to Ko Chang in Trat province. www.pttaxiservice.com/page_3003.php?page=03 The price probably does not change if they drop you off sooner - ask them.
A random taxi from the airport may not be suitable for a drive of 4 hours. Taxis use natural gas or petroleum gas with a big tank in the boot. The standard drill when going to Pattaya less than 2 hours away is a pit stop for topping up gas.
thanks very much for the prompt reply