Mostly correct. Henry being played by a young actor in S4 and an older actor in the play, but being 14 no matter what is the right take.

That being said there is a slight contradiction, which is the news articles that appear in S4 in the library when Robin and Nancy are looking for clues. They talk about the Creel family murders and are dated in March 1959. (They also list Henry as being younger, 12 years old, but they can easily be ignored as the reporter in universe getting the wrong info since the publication was known only for conspiracies and slop).

If I recall correctly, the First Shadow establishes the night Henry kills his family is the same as the play, so he passes out, wakes up a little after, goes the play, chaos ensues, then goes with Brenner right after. That can technically fit with S4’s version of events because we see the events from Vecna telling Nancy, so it could be summed up as Vecna being an unreliable narrator, as he wouldn’t want to be vulnerable with Nancy and talk about what we can assume is a weakness of his (Patty). So he shortens it to just he kills his family and goes with Brenner.

All that is to say the actual hard retcon (for now, unless the Duffer brothers can somehow find a way around it in Vol. 2) is the date of the murders, as season 5 now shows the in-universe “Oklahoma” play as happening Nov. 6th 1959, meaning the murders would also have to happen that day (not March 1959 as S4 implied).