Within the military there are certain jobs and certain roles and titles given to different types of soldiers and officers. The role of the G4 within the coordinating staff system is to monitor, assess, and coordinate personnel administration. A G4 is usually known as a chief of staff for logistics engineering.
This is spread over all branches of the military including Army, Marines, Navy, and Air Force. Each staff level G1, G2, G3, G4, and G5 is concerned with one broad field of interest within the military. G4 is specifically rooted in logistics. The rank above G4 is the highest and is concerned with civil-military operations.