Is your chain too tight? 4 inches is close to correct, at just over 100mm. 1.5" of free sag indicates that the spring is on the stiff size for someone of your weight though. I'd make sure you have your sag measurements correct, set it to 100mm of race sag and re-check your free sag. If you free sag is over 30mm consider a softer spring. Of course, getting the shock serviced every season or two is a good idea. Don't forget to adjust the clickers as well. Start with factory settings and go from there.
good luck