I have been requested to install a drip leg/sediment trap on the gas supply to the water htr. The heater was installed in 2008.is this a code requirement now if it is does it need to be installed on a4yr.old installation

Who requested? It does help to trap any small particles from the gas line from clogging the pilot valve. It is relatively easy to do. At some point you should have an ell pipe fitting from the input line to the gas valve or possibly at the shut off valve. Install a black iron "T", most likely 1/2" with a side of the " with a side of the " down. Screw a 1/2" nipple, 6" down. Screw a 1/2" cap. Dope all the fittings and leak test.