Is it possible to disable features so that C4 runs on graphics cards older than the minimum requirements?
To an extent, yes. C4 can be made to run on any graphics card that supports fully programmable pixel shaders, but the requirements are a little higher for terrain and shadow mapping. For all engine features to be available, the minimum requirement for Nvidia hardware is a GeForce 6600, and for ATI hardware is a Radeon X1500. Without terrain and shadow mapping, the minimum requirement for Nvidia hardware is a GeForce FX 5600, and for ATI hardware is a Radeon 9500. See the C4 Engine Graphics Hardware Compatibility.