Welcome to iRV2.
Simple answer , yes you can go down to using 4 batteries ( I'm assuming 4; 6volt batteries
) provided they are hooked up correctly to provide 12volt power to the coach.
Many coaches out there using only 4 , including my own.
However, if you are in a situation , where you are parked for a lunch break , lets say , and your coach is set up with an inverter that powers a major draw item ; like the microwave; using the microwave could overheat the 4 batteries by a very rapid draw down and shorten their lifespan.
Important to understand all the , possible situations , that the manufacturer was accounting for , when the 8 batteries were installed in the first place.