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

1. Variations in topographic height are compensated by lateral variations in crustal density.

2. The Earth's crust floats at different heights due to differences in density, not thickness.

3. The thickness of the Earth's crust is uniform, and variations in elevation are explained by crustal density.

4. Lower-density materials in mountains extend deeper into the mantle, creating equilibrium to support elevation.