Most of the car engines are fiited on three vibration isolators (Mountings) to isolate the vibration from engine to vehicle chassis. What will happen should we go fo 4 mounts?

I am pretty sure the engineers have got it figured out that 3 is the best number for engine mounts. My guess is adding mounts or making them more rigid will cause unwanted vibrations or noise.

Engines have to move a certain amount. Adding a lot of engine mounts would probably make vibration worse. The current set up has been developed over a 100 years and works pretty good.