A closed cylindrical container with base radius 4 cm and height 9 cm (container wall thickness is negligible) contains two iron spheres of equal radius. The maximum radius of the iron spheres is \_\_\_\_ cm.
A closed cylindrical container with base radius 4 cm and height 9 cm (container wall thickness is negligible) contains two iron spheres of equal radius. The maximum radius of the iron spheres is \_\_\_\_ cm.