This volume contains selected papers for the Parcella '88, the Fourth International Workshop on "Parallel Processing by Cellular Automata and Arrays" held in Berlin from October 17 to 21, 1988. The profile of the Parcella workshop series is focused on problems of processing by regular structures, i.e. their "flexibilization" or adapting to "irregular" algorithms and, closely related to this, on the "regularization" of algorithms for their embedding into regular structures. It seems that these problems will have an increasing priority within the list of central problems in parallelization and will determine the profile of Parcella for the next years.