Directed multicut is W[1]-hard, even for four terminal pairs

We prove that Multicut in directed graphs, parameterized by the size of the cutset, is W [1]-hard and hence unlikely to be fixed-parameter tractable even if restricted to instances with only four terminal pairs. This negative result almost completely resolves one of the central open problems in the area of parameterized complexity of graph separation problems, posted originally by Marx and Razgon [SIAM J. Comput. 43(2):355–388 (2014)], leaving only the case of three terminal pairs open. The case of two terminal pairs was shown to be FPT by Chitnis et al. [SIAM J. Comput. 42(4):1674–1696 (2013)]. Our gadget methodology also allows us to prove W [1]-hardness of the Steiner Orientation problem parameterized by the number of terminal pairs, resolving an open problem of Cygan, Kortsarz, and Nutov [SIAM J. Discrete Math. 27(3):1503-1513 (2013)].