Can you load a 12.4 version of the IOS onto a 4006 chassis that has been upgraded from a Sup II to a Sup IV? 4006 has come to the EOS and it looked like the option was to upgrade to a 4506 IOS.
There is only one explanation for that. It seems you weren't typing the filename from the image correctly on the boot system command.
The image may contain "_" or a "-" that you may have missed. What I usually do, is dir bootflash: and copy the image name as displayed in the CLI onto the command.
I never trust myself on typing the image name correctly :)
I haven't seen a 4006 in years :)
4006 is current EOL so it's very hard to find any documentation on it but I 'assume' the Sup should load the installed IOS.
I don't have any hardware to duplicate your environment, sorry..