This is a strange little deal that hinges around whether or not you own BOTH a PS3 and a PS4.
If you own both consoles, you can buy the PS3 version of Final Fantasy XIV for about a fiver, then upgrade to the PS4 version for free. This is naturally much cheaper than buying the PS4 version from the off... but if you don't own a PS3 and want to get involved with Square's much-improved MMO, ShopTo are currently touting a whopping £9 saving. Note that you'll have to start paying a monthly subscription once your included time is up, though. Thanks to cerberus1 @ HUKD!