There will be no more BIOS updates for the 4600.  Updates usually do not continue past the end of sales for any given model, once most stability issues have surfaced and been resolved. What kind of error are you getting when trying to install it?
I'm using a Dell Dimension 4600 to test the Consumer Preview of Windows 8 (Yes, I know it's a dinosaur).  The Consumer Preview is working fine. However, the Release Preview will not install. I suspect it is because the BIOS is too old. (The version that is currently installed is S12.)  Are there any moves afoot to upgrade the BIOS and expose the NX / XD bit in the BIOS security settings?   Or, is there a work around to enable installation of Windows 8 Release Preview on this level of machine? Thanks
Windows 8 - BIOS update for older computers?