Using 4 bits, one can represent 24=162^4=1624=16 different values. This is because each bit can have two possible states (0 or 1), so with 4 bits there are 2×2×2×2=162\times 2\times 2\times 2=162×2×2×2=16 possible combinations. These values range from 0 to 15 in unsigned representation.
If the representation is signed (using two's complement), the values range from -8 to +7, still totaling 16 different values but distributed between positive and negative numbers.
