Beta Phase 4 starts:
You import your 1.0 data, and start playing and doing stuff. You can't change your name or appearance.
You play and level and get loots all the way to end of phase 4, when Official launch starts.
As long as no game breaking exploits are found that would allow stuff like leveling to 50 in an hour, duping items, or generating massive amounts of gil, character data from Phase 4 will carry over into official release intact.

So for instance, 2 weeks into phase 4, they notice that there's an NPC that buys an item for more than another NPC sells it for, and the economy tanks as that infinte gil floods the market. At that time, they would likely bring the server down, fix the issue, wipe character data, and bring it back up. You could then create a new character or re-import your 1.0 character and keep going.

Once official release happens, you would log in with your Phase 4 or 1.0 character, and go talk to an NPC that would handle changing your name and appearance.

At least that is how I interpreted everything the devs have said over the last few months of updates.