This book constitutes the refereed best selected papers of the 4th International Workshop on Parameterized and Exact Computation, IWPEC 2009, held in Copenhagen, Denmark, in September 2009. The 25 revised full papers presented together with 2 invited talks were carefully reviewed and selected from 52 submissions. The topics addressed cover research in all aspects of parameterized and exact computation and complexity, including but not limited to new techniques for the design and analysis of parameterized and exact algorithms, parameterized complexity theory, relationship between parameterized complexity and traditional complexity classifications, applications of parameterized and exact computation, implementation issues of parameterized and exact algorithms, high-performance computing and fixed-parameter tractability.