I've always understood it to mean that come phase 4, our progress and characters will be saved. Afterall, they won't delete a 1.0 character, so 'Circumstance may necessitate the deletion of character data' would only refer to progress in this case, not the actual character/appearence. Basically, worst case would be they have to roll-back a server if they think there was a major issue/exploit etc.
Also, according to the roadmap, for phases 1, 2, and 3 it was explicitly stated that characters would be deleted at the start of the next phase. And usually it was stated in the phase before it
IE: -"Phase 3 characters will be deleted at the start of phase 4." is listed under the Phase 3 portion of the roadmap, not phase 4. Using this logic, if they planned on not carrying over character progress, they would have added that into phase 4's roadmap.
In addition to this, the little information that has been provided by SE on ARR servers and how we get on them also implies, that our characters might be 'locked' to Phase 4 servers. It's vague, but considering they are planning on letting anyone with an Account have Beta 3 access so we can communicate with eachother, it is plausible that they plan on having us select our servers pre release. (Especially since anyone with a character created pre ARR release, will not be able to roll that character on a New Character server. Basically, this would force anyone who got 'early access' via Phase 4 to keep their 'early' progress on an established server. Only new characters,could start on a new server at official release.)
Yes, I know this too is very vague and will need a lot of clarification, but I take all this information to mean come phase 4, character progress is saved in part, because we're on the servers we'll be on at release to save it.
Basically, for now, I'd plan on having data saved, and playing as such...but not getting my panties in a bind if they had to reroll my character for any reason. Most important to me is to 1. Get on a server with my friends. 2. Help my hubby and friends who are new to the game, get started.