These are the proceedings of the 4th Annual Workshop of the EURATOM FP7 Collaborative Project \"Redox Phenomena Controlling System\", held in Karlsruhe (Germany) 23rd to 26th January 2012. The project deals with the impact of redox processes on the long-term safety of nuclear waste disposal. The proceedings have five workpackage overview contributions, and 24 reviewed scientific-technical short papers. The proceedings document the scientific-technical progress of the fourth project year