Both functions (2^x and 4^x) have exponential growth, but their exponential growth is not linearly related.
2*(2^x) != (4^x)
2*(2^x) != (4^x)
Take, for example, 32bit integers vs 64bit integers (unsigned for simplicity). Two 32 bit integers can represent exactly the same number of combinations that a single 64 bit number can. Sure, there's an exponential number more combinations in a 64bit integer than a 32 bit, but the number of combinations is not how the storage capacity is measured.