(Unfortunately it did not work under Slitaz 4.0rc2 as well, same result: gunzip: invalid magic)
But this time it worked, with an error message, but finally I got a directory ~/kompozer. With an install.sh-script in it. I applied it, it installed into /usr/lib/ and /usr/bin - but it did not work yet, claiming not having found a mozilla runtime directory. - But keep in mind: I am on 3.0 now.
To install this devel-4.0-iso that works on this machine will now take extra time. Stay tuned, please!
Plus: on this machine rc2 displayed the same problem.
(I booted using all 4 rootfs-files, with nomodeset and without: I got a X-screen, on white-ground, with one command line. Lsmod showed that no nouveau module was loaded. When I used tazw and then selected ,,nv" immediately the screen got black and I could escaping only with a hard reset: nothing reacted anymore. Now I still have 2 successful ,,4.0"-variants: this devel-flavour and the one that you prepared, do not know how, though. All the rest result in black screens)
Now I download it a 3rd time. Because, you know, that close the goal...