looking at boat with 1999 502 magnum MPI. compression on all cylinders is between 150 and 160 except for #4 which is 135. Is this a concern? 135 hours on motor.

no, you are ok and within specs but I would do a leakdown test on that cyl just to be on the safe side!!!

Stock 502 mpi's are notorious for one or two low cylinders,they use J/E pistons from the factory with metric low tension rings. My 2000 (for the 1 month in its life it was stock) used 1 quart of oil from day one every 5 hours. I complained to the local baja dealer and they did a leakdown,it had one weak cylinder. They pissed around for 4 months,let it freeze solid in there parking lot and i finally got disgusted with them and took it from them and pulled the motor myself then the games began,Smitty