You didn't really say what your network topology is, but I'm assuming that you have a DSL modem, and 4 computers all plugged into a hub or switch, and there's no router.
I posted in Linux Software (I think, or newbie) about how to set up your smb.conf file, there's an example one there that should not need much tweeking for your setup. If you want to secure your network using the hardware you have, you should buy a second NIC for the Linux box, and configure iptables and IP masquerading on it... there are a bunch of example scripts floating around this forum. Anyway basically once you have the second NIC installed, set up the connection like this:
| DSL Modem |
| Redhat 9 |
| Hub/Switch |
then the rest of the puters off the hub. Configure iptables to only allow ftp, ssh and whatever from your internal LAN IP range. DHCP and PPPoE should not be a problem, but I've only set up DSL on OpenBSD. Also you could run dhcpd on the internal interface of the Linux box so that you can essentially plug any computer into the network and have internet access for it.
HTH, if you're more specific with your problems it's easier to give you specific advice.