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I'm launching a not-so-critical thing in 2 or 3 months, currently using Bootstrap 3.

Is it safe to adopt 4 right now, to be more future-proof? I don't mind bugs that eventually will be fixed but is the API stable enough?

Definitely not. We're in the alpha stage. Things won't be locked in until maybe beta, but for sure the release candidates.

you might want to consider using bootstrap-sass package, so you can still use sass while preparing for bootstrap 4.

bootstrap-sass is an official bootstrap project.