The pressure rating would be for its ability to check backflow. I assume you don't have anything backflowing from your undercarriage above 400 PSI. That said, you probably need to worry more about the maximum flow rate through the valve, for example I tried a 1/4" Fluid Controls HP check valve on a medium pressure spot free line to the bay, and the flow of water was so high it broke the plastic tabs that hold the poppet in place and it went downstream and got jammed in the fitting. A 3/8" one in the same place worked fine.