What should I expect after my height and weight flag is lifted? Will I still be able to get the promotion I was denied?

A enlisted promotion that was wrongly denied due to an erroneous flag has a remedy for correction in the regulation.
AR 600-8-19
3–30. Rules for reinstating Soldiers to recommended list
a. A Soldier promoted in error to the ranks of SGT or SSG will, when otherwise qualified in accordance with paragraph 1–10, be reinstated to the list at the same time the order is revoked and treated as if never promoted.
b. A Soldier removed from a list and later exonerated of the basis that caused the removal will be reinstated. To be exonerated, the action that caused the initial removal must have been erroneous or should not have been imposed so that the Soldier is free of any wrongdoing.
c. If the Soldier was eligible for promotion prior to reinstatement, the DOR and effective date of promotion will be the date of original eligibility.


However, you were not in a promotable status at the time of the flag, so I do not believe the reg has a remedy for your specific situation.
Your unit may elect to board you immediately regardless of the next "scheduled" promotion board in effort to "right" the wrong but I do not see anything else that can be done except move forward.

Who told you the flag erroneous? Assuming it was your chain-of-command, then you should be fine. However, you need to make sure the flag is removed first. Also, have you already been to a board? Unless things have changed in the last few years, E1-E4 promotions are company-level approved, completed with a simple 4187 coded by your battalion S-1 typically. So in short, yes, it can be backdated but it will be up to your Commander and S-1 to make sure that happens. Talk with your platoon sergeant/section leader/NCOIC to make sure they know your flag was erroneous and specifically ask them if your promotion will be backdated. E1-E4 should not be problem, I myself had my own E4 promotion backdated due to an error many years ago. Once you get to E5 it gets bit more complicated, but regardless, the ability to backdate promotions exists in part for situations like this (along as your command agrees).

No promotion is guaranteed including E4. You must be effective in your job and demonstrate that removing your flag might only allow you to return to the bad habits that caused it in the first place.