First why use the switch when you have 4 available ports free on the router?!?!!?!?
2nd, no worries about no mumbo-jumbo connecting to your router if you secure it using a WPA2 key and you already said that you disabled the wireless on your router. How could it be possible?? No physical connection to your router, wireless off, it's not possible. If you want an extra layer of security you can add macfilters for every one of the pc's. The DHCP server is pretty good for this router. I don't know how one could connect to your router if you disabled the wireless function.
What revision is your router? DIR-615 rev C or D? If it's C take a look at the emulator! It's pretty straight forward.
http://www.support.dlink.com/emulators/dir615_revC/310NA/index.htmYou are very determined to have static ip configuration.
Go to 192.168.0.1
Manual Internet connection
My Internet Connection is: Dynamic
Network settings: Disable DHCP server
Setup mac filters: Advanced->Network Filter->check 4 boxes and add the coresponding mac addresses. This after you have valid connections for all the pcs so they will be listed. If not copy the mac addresses and enter them manually.
DON'T FORGET TO SAVE.
Then from every pc's network settings set-up consecutive static IPs.: 192.168.0.100, 192.168.0.101, 192.168.0.102, 192.168.0.103 from the 192.168.0.0 subnet.
DNS: 255.255.255.0
Gateway:192.168.0.1
DNS: leave it automatic or you can add Open DNS (208.67.220.220) and Google DNS (8.8.4.4)