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I have some simple advice: if those instructions aren't working for you, then they either misstate something or leave something vital out. Linux 'documentation' is often that crudely unreliable, unfortunately.
Before you reboot and try to connect to the new Arch Linux on the Pogo Plug 4, do you set the IP address on it to come up at a fixed IP address? do you set its Linux eth0 to be listening and active? if you don't, it won't come up at any IP address, or it will come up - as assigned by DHCP perhaps - at an IP Address that you cannot forsee and must discover by guesswork or examination and inference.
What have to tried to do, to examine and infer the IP identity of the Pogo Plug device on your network ? Reading the DHCP Assignments tables of your local router box (if the Pogo device is connected to it with an Ethernet cable, especially) would be the most likely way to go with that.
(And if the Pogo device had a 'console' connection, that would be the best thing to examine it by, since it would show scrolling bootup/startup messages including info about network status. That's an IF, since I doubt that it has.)