It was also a minor security revision, not like WebKit, or Darwin kernel saw any changes. What did it fix that effects/delays a OS that won’t ship for months? As you said Watch OS is ready for DB4.
You are assuming, perhaps incorrectly, that the revision has anything to do with the timing of beta 4. It might simply be that Apple allows 3, rather than 2, weeks between betas 3 and 4.