if so, what could I do to help one to write a hack ?
the first thing that is needed for your hack(afaik) is root. in a linux/unix system root is god. key to the kingdom(with the exception of a few things..). root access allows read/write operations on systems that are marked for read-only to the end user typically. although i am not familiar with any existing 404 hacks(i am a 605 owner) maybe they exist? someone chime in if they do.
if not that is where the problem starts, my understanding is the 404 does not have wifi- which rules out GFT/GFT2/3 which(afaik again. lol) all make use of injecting code into system calls to various wifi functions. most likely if these types of attack vectors(users changing files or inputs from what was expected by developers) exist in x05 software they may also exist in legacy software.
1. download linux if you dont use it already- personally i use ubuntu as its pretty easy and has a big community that supports it well, but there are many good variants of GNU/Linux/BSD/*nix like/posix compliant(osx to some extent... )http://www.ubuntu.com/
2. download the GPL sources provided by archos (think theres a x04 link on the openPMA wiki)http://www.openpma.org/gen4/Sources
3. learn c and/or ARM assembly(this may be a stretch, and may not be required, but certainly wouldn't hurt)http://en.wikibooks.org/wiki/C_Programminghttp://en.wikipedia.org/wiki/ARM_architecture
4. download and open a hex-editor, start looking around your system--see what files can be trivially modified(i.e. no check-sum, no signing)
sudo apt-get install ghex (in the terminal of your new ubuntu!)
5. find or buy some good decompilers that support ARM(a popular one is IDA pro)
6. report your findings back here and...