The problem is that sun has changed the name of the jdk-1.4.0 to download, it used to be called j2sdk-1_4_0-linux-i386.bin but is now called j2sdk-1_4_0_01-linux-i586.bin This has the effect that the emerged fails, im guessing the fix would be to change the ebuild file and possibly also the cheksum and then test it.
Check my bug #3371, It is not the same file, it is a new version.
Again, this shite is something you asked for, Matt :)