1. If you buy the game on Steam, it will be linked to your Steam account immediately. The CD Key itself however does not need to be registered immediately, and you can register it to your account two weeks later (or so on) if you wish, however you will not be able to play it until then (obviously).
2. I don't know the differences between them, but if you link them to the same account then I suspect you'd get all the PS4 pre-order goodies when you play on the PS4, and any duplicate items will simply be negated. I'm not 100% of this though.
3. AFAIK, no. There's simply US and JP servers.
4. Again, I'm not sure, but I don't think so. I'm in Australia which often flits between US/EU depending on games/companies etc, but I've not noticed anything linking my account to EU or US specifically.