There are a few ways to get most of the modern linux distros running on a usb flash. Almost all of them have a way to save data. I assume you mean on the usb flash. Most can also save on the built in hard drive.
The 4 gig is good for many of the ways at www.pendrivelinux.com
and each distro's site for more idea.
The 16G can easily handle any common distro installed natively using an ext2 filesystem and have tons of room left over. I tend to use 8G for many of my flash installs. It gives me about 4G left over on common big distro install.