Introduction:
In this scenario, we have a stationary body of mass m being lowered onto a moving platform of mass M. The platform is moving at a speed of 4 m/s. We need to determine how far the body will slide along the platform.

Key Variables:
- Mass of the body: m
- Mass of the platform: M
- Initial speed of the platform: 4 m/s

Principle of Conservation of Momentum:
The principle of conservation of momentum states that the total momentum of a closed system remains constant if no external forces act on it. In this case, the body and the platform form a closed system, and the total momentum before and after the body is placed on the platform will remain the same.

Analysis:
When the body is placed on the platform, it will start moving along with it. Let's assume the body slides a distance x along the platform.

Applying Conservation of Momentum:
Before the body is placed on the platform, the momentum of the system is given by:
Initial momentum = mass of body (m) * 0 + mass of platform (M) * velocity of platform (4 m/s)

After the body is placed on the platform, the momentum of the system is given by:
Final momentum = mass of body (m) * velocity of body (v) + mass of platform (M) * velocity of platform (4 m/s)

According to the principle of conservation of momentum, the initial momentum and final momentum should be equal:
Initial momentum = Final momentum

This can be written as:
m * 0 + M * 4 m/s = m * v + M * 4 m/s

Simplifying the equation:
4M = mv + 4M
mv = 0

Conclusion:
From the equation, we can see that the mass of the body (m) multiplied by its velocity (v) is equal to zero. This implies that the body does not slide along the platform at all. The body remains stationary on the moving platform, and its velocity is zero. Therefore, the distance the body slides along the platform is zero.