The Workshop on Parameterized and Exact Computation (IWPEC) is an - ternational workshop series that covers research in all aspects of parameterized and exact algorithms and complexity, and especially encourages the study of parameterized and exact computations for real-world applications and algori- mic engineering. The goal of the workshop is to present recent research results, including signi'cant work-in-progress, and to identify and explore directions for future research. IWPEC2009wasthefourthworkshopintheseries, heldinCopenhagen, D- mark, during September 10-11, 2009. The workshop was part of ALGO 2009, which also hosted the 17th European Symposium on Algorithms (ESA 2009), the 9th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2009), and the 7th Workshop on Appr- imation and Online Algorithms (WAOA 2009). Three previous meetings of the IWPEC series were held in Bergen, Norway, 2004, Zu ]rich, Switzerland, 2006, and Victoria, Canada, 2008. At IWPEC 2009, we had two plenary speakers, Noga Alon (Tel Aviv Univ- sity, Israel) and Hans Bodlaender (Utrecht University, The Netherlands), giving 50-minutetalkseach.ProfessorAlonspokeon"ColorCoding, BalancedHashing andApproximateCounting," andProfessorBodlaenderon"Kernelization: New Upper and Lower Bound Techniques." Their respective abstracts accompanying the talks are included in these proceedings. InresponsetotheCallforPapers,52papersweresubmitted.Eachsubmission was reviewed by at least three reviewers (most by at least four). The reviewers were either Program Committee members or invited external reviewers. The ProgramCommittee held electronic meetings using the EasyChair system, went throughthoroughdiscussions, andselected25ofthesubmissionsforpresentation at the workshop and inclusion in this LNCS volume.