Measuring the inconsistency degree of an inconsistent knowledge base is an important problem as it provides context information for facilitating inconsistency handling. Many methods have been proposed to solve this problem and a main class of them is based on some kind of paraconsistent semantics. In this paper, we consider the computational aspects of the inconsistency degree of a propositional knowledge base under 4-valued semantics. We first analyze the computational complexity of computing the inconsistency degree. As it turns out that computing the exact inconsistency degree is an intractable task, we then propose an anytime algorithm that provides tractable approximation of the inconsistency degree from above and below. We show that our algorithm satisfies some desirable properties. This algorithm is additionally parameterized by a precision threshold ε, by which we can control the efficiency of the anytime algorithm.