Apple stop signing firmware 4.0.1. Thus, you can only officially upgrade to 4.0.2. You will get error "this device not eligible..." if you try to install any other firmware.
Only way you can install 4.0.1:
1) Are you currently on pwned 3.1.3 JB? If yes, you can create / download a custom 4.0.1 firmware and install it.
2) You have 4.0.1 SHSH hash saved. Edit your host file point Cydia or run TSS server option in tinyumbrella. Download official 4.0.1 firmware somewhere (google). Then, restore using that file through iTunes.