You can use 5.5 (Brigantia), and have an /etc/apt/sources.list that looks
like this:
deb http://oldarchive.trisquel.info/trisquel brigantia main
deb http://oldarchive.trisquel.info/trisquel brigantia-updates main
deb http://oldarchive.trisquel.info/trisquel brigantia-security main
But it's been unsupported for some time, though. It's probably affected by
Shellshock, for example (though you can easily add deb
http://archive.trisquel.info/trisquel toutatis-security (or updates?)main and
take the new (and presumably fixed) version of Bash from there.
