This is my guess, but official clarification would be nice:
You might be able to keep the character it's self you create in phase 4, progress and whatnot are unlikely to carry over. They do state there may be early access offered between the end of Beta 4 and release, but typically no beta progress carries over to the live version.
Where my reasoning comes from:
Q: When characters are carried over to ARR from Version 1.0, will we only be able to change our character's appearance one time at launch? Can we use our one time change during phase 4 of the Beta as well? Or will it be possible to do it twice, during phase 4 and at launch?
A: Please keep in mind the Beta is a testing period meant to check data. All character data has been examined thoroughly, and there are no bugs. However, I would like our players to understand that they will not be able to change their character's appearance until ARR is officially released. The problem with allowing this before then is that if something goes wrong when changing a character's appearance, it'll be difficult to determine if whether it was the result of a bug, an error with the data transfer, or some other process that occurred between beta and release.
they give little hints about not changing the character data until release, just in case there is a problem. Now ganted they are just talking about apperance here, but if apperance is that big of an issue to change and save, then progress may be a bigger issue.