Actually, some of that might be the standard to which the vibration system is maintained, and some to the nature of the aircraft.
You might want to have the mechanics toss on the vibe gear and take some measurements to compare to its history. Doesn't hurt to ask.
Generally, the job it does and the weight it operates at don't affect the 4P (20Hz) very much. I'm going to bet it has more 1P (5Hz) than it should. You can almost count 1P and it bobbles you more, 4P is more like a rumble on the floor and in the seat.