V4 is the instantaneous velocity after the collision of A and B.. Obtained by conservation of momentum.

When moving upward, the free gravity acceleration of object A on the inclined plane is, gsinθ.

Therefore, only when the acceleration of B is greater than gsinθ will A and B begin to separate, so only when the spring instantly pulls down the object B (that is, at the position of the free length of the spring) will A and B begin to separate.

The highest point is the intersection of the vertical line passing through the center of the circle and the arc, not the other end o of the arc.