The fourth GMA workshop took place on the 9th and 10th September 2015 at the offices of GDG in Dublin, Ireland. This workshop was the second workshop focused on the rail network case study in Croatia. The GMA aimed to identify the scenarios to be investigated in the risk assessment, and to isolate those events which are deemed to be most important and treated in more depth. Following on from the GMA workshop in Zurich in March 2015, this workshop had two primary goals; (i) To fully specify the structure of the models being used within the case study and (ii) To assess the conditional dependencies in the morphological model in order to turn it into an (input-output) inference model. The workshop was attended by the facilitator, RCAB, representatives from the relevant work packages and from the chairman of the advisory board and PSJ who provided input and insight to the workshop from a Stakeholder’s point of view. The workshop resulted in fruitful discussions in relation to the most appropriate way to progress the rail case study, with the findings of the workshop indicating that it would be appropriate to develop a Bayesian Network model to have a better understanding of the relationship between the different variables influencing the risk to the rail network in Croatia.