You only need disk 1 unless you plan on installing gnome or kde. After the install, I would mount the four isos via loopback. It saves burning lots of disks that way. For example, I have in my fstab:
/usr/iso/slack1 /mnt/slack1 iso9660 loop,auto,users,ro 0 0
/usr/iso/slack2 /mnt/slack2 iso9660 loop,auto,users,ro 0 0
/usr/iso/slack3 /mnt/slack3 iso9660 loop,auto,users,ro 0 0
/usr/iso/slack4 /mnt/slack4 iso9660 loop,auto,users,ro 0 0
where /usr/iso/... are symlinks to the isos elsewhere.
The second two disks are source disks, which are handy because they come with build scripts that you can edit for your processor, and then you get tarballs of programs optimized for your machine, but you don't need the disks for installing. (Also, there are extras like bittorrent and checkinstall on disk 3, I think)