I don't think that's right. Assuming we had 5 characters to encode data,
with base2 we get 2^5 = 32 possible combinations
with base4 we get 4^5 = 1024 possible combinations
with base2 we get 2^5 = 32 possible combinations
with base4 we get 4^5 = 1024 possible combinations
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.