Four blocks of masses M1,M2,M3 and M4 are placed on a smooth horizontal surface along a straight line as shown above. It is given that M1>>M2>>M3>>M4. All the blocks are initially at rest. M1 is given initial velocity v0 towards right such that it will collide with M2. Consider all collisions to be perfectly elastic. The speed of M4 after all collision are over is