The total number of boolean functions, which can be realised with four variables is
The total number of Boolean functions, which can be realised with four variables is 16.
The values a, b, c, d of the function f can be either a 0 or 1. Since there are 2 choices for 4 values, there are a total of 16 = 24 = 222 different functions.
A Boolean-valued function is a function of the type f : X-B, where X is an arbitrary set and where B is a Boolean domain, that is a generic two-element set, whose elements are interpreted as logical values, for example, 0 = false and 1 = true, that is, a single bit of information.