G-4, also known as Marine Research and Development Institute (海軍技術開発局 Kaigun Gijutsukaihatsu Kyoku?) is a Marine branch that is located in the Grand Line . It was first mentioned by Mest, a former prisoner in it. It was described by him as "a living hell". However, as the name suggests, it is a research facility. It utilizes captured pirates and what the Marines consider "good specimens" to test medicine, drugs and conduct physical and chemical experiments.