Gravity does not decrease beyond 4500 meters below sea level. In fact, gravity increases as you move deeper into the Earth. The reason for this is that the Earth's mass is not distributed uniformly, and it becomes more concentrated towards the center.
Gravity is a force that attracts objects with mass towards each other. The force of gravity depends on the mass of the two objects and the distance between them. When you are closer to the Earth's center, there is more mass below you, which means there is more gravitational attraction pulling you downward.
As you descend below the Earth's surface, the increasing mass of the Earth beneath you causes a greater gravitational pull in the downward direction. However, the increase in gravity is not uniform, as variations in the Earth's density and mass distribution can affect the local gravity field.
The notion that gravity decreases beyond 4500 meters below sea level might be a misunderstanding or a misinterpretation of some other phenomenon. However, in reality, gravity becomes stronger as you move deeper into the Earth.