I want to put my amp under the front passsenger seat. Theres probably about 4 inches height under there. Will it be ok?

 

I would try to find a better place than under the seat. An amp needs to breathe to keep from overheating. Placing it under your seat can restrict adequate air flow and cause the amp to run hot. Heat and vibration are the 2 worst enemies of electronic components. While there isn't a lot you can do about the vibrations in an automotive environment you CAN do something about keeping the amp cool.