There is nothing wrong with the hardware except that it is different. The A4000 has a different IDE controller than an A1200 and thus needs a different driver.
You've got a pre-installed OS which has been patched by some stupid guy in such a way that it only works on an A1200.
If you revert to an unpatched OS 3.9, it will run on both machines because it contains all required drivers. The drivers of OS 3.9 do not exist as seperate files, but they are packed together in a file called Amiga OS ROM Update which resides in the Devs directory. Upon boot the SetPatch command will load this file and install the machine-specific drivers in memory. LoadModule is not part of the OS and is not required.