From all indications (and personal experience), Pogoplug no longer allows account holders to remotely activate SSH on their devices. The only way then to change stock u-boot or upgrade u-boot so you can run Arch or Debian on any stock Plug (V4, E02, Pro, Biz, etc.) is to connect and communicate via serial console. In the case of the V4 series, that requires soldering three leads onto the motherboard. If you are not a hands on type, you may wish to aquire an E02 which can be connected to with a CD-ROM cable. Regardless of which Plug you try to upgrade and/or modify, it still requires opening up the case.
Sorry if this was not the news you were hoping for.