@Donnie-D of course there's a lot more changed than
text-center (from the link above). There are some explanations in that link on some under the scene stuff that have changed, and there's a mention of the class names that have been renamed (this, for example).
Now, I don't have much info about this, but you can search online for what has changed (bootstrap 4 vs 3, for example).
And it seems that resources/blogs online advise to use the newer version (4), but the class names in v4 may still change, cause it's still in beta.