Identify which of the following statements accurately reflect the principles of John Henry Pratt's theory of isostasy.

1. Pratt's theory suggests that variations in topographic height across the Earth's surface are compensated by lateral variations in crustal density; regions with taller topography have a less dense crust.

2. According to Pratt, the Earth's crust floats at different heights on the mantle not because of varying thickness of the crust but due to differences in density.

3. Pratt proposed that the thickness of the Earth's crust is uniform, with variations in elevation and mountain building being explained by variations in crustal density and not by crustal roots.

4. The concept introduced by Pratt asserts that lower-density materials in mountainous regions extend deeper into the mantle, creating an equilibrium state that supports the elevated topography.