Newest one does not compute. Each distro has its own stable testing and unstable distro. Red Hat sells its stable distro as Red Hat Enterprise Linux while giving away its testing and unstable versions as Fedora. Debian is currently moving from Etch as the stable distro to Lenny, which makes Sid, the current unstable distribution the new testing distro (and though I'm typing this on Lenny/Sid, I don't know what the new unstable distro is.
Unstable essentially means as soon as a package is available, it becomes part of the distro. It may cause crashes and other hangups. If it doesn't, or some developer reworks it so that these crashes and hangups don't happen right away, it's moved over to testing for a period of time. If no big problems crop up, or it''s fixed, it becomes stable.
Slackware is the oldest distro. It's essentially a vanilla linux. It generally does not come with the Gnome Desktop (hasn't for years) but it is the one desktop where you can essentially install any package from anywhere without giving it a second thought. If you ask for help people will tell you how to recompile it. It's the hardest to update through the repositories but you can do it.
Gentoo is one of the most difficult and painful to install. It also uses rsynch for the updates, which essentially means every time you run portage, the update program, you are getting the most up-to-date stable or testing version available. It would be the equivalent of installing Gutsy Gibbon and today running Hardy Heron without any effort to make the change.
Debian is very closely related to Ubuntu, but Ubuntu is tweaked to make it friendlier to new users. You decide what desktop you want to install -- but for example I have both KDE and XFCE4 installed on this Debian box. Fedora runs selinux, making it the most secure version out there (selinux was developed at the National Security Agency).
The whole point of Linux is it is about choice. I would go to distrowatch, browse the distros, download two or three and try them in order -- my current Gentoo laptop used to run Fedora Core 9, while my retiring gentoo laptop has a debian etch drive. Most of them are available as Live CDs anyhow. Yes the 4 gig jump drive should be able to handle them (but don't use Gnome -- I don't like it but that's irrelevent: it takes up a lot of disk space. KDE takes up somewhat less and XFCE takes up a LOT less. For a 4G jump drive I would go with a Debian system LIKE Debian, *buntu or Linux Mint and XFCE desktop: Debian systems tend to have a light footprint anyhow, which is why Ubuntu is so closely tied to Debian some four years after the fork. Otherwise I'd say, try them all and have fun.