It is clear that the four realms are not independent, but rather intrinsically interconnected. First of all, the spheres extend into one another, with maximum overlap at the surface. Secondly, all physical processes on Earth involve all realms in one way or another. For example, weather systems are generated in the atmosphere, but depend on factors such as evaporation of moisture from the hydrosphere and from plants in the biosphere. Moist air is then transported from one place to another by air currents that depend on the nature of the surface - the lithosphere.
Volcanic eruptions occur in the lithosphere, but affect the other realms substantially.
This is a very thin layer, it lies within 100 m or so from the surface. This is the place where life on Earth is most protected and enjoys most stability. Geosphere - all 4 spheres taken together. It therefore extends from the bottom of lithosphere to the top of atmosphere. Geosphere could be considered the object of study of physical geography as a discipline.