The reason I asked this question is because if Opus 4 shares a base model with Opus 3, there would have been at least 2 distinct pretraining phases, the main one with all the stuff up to August 2023, and then an update with probably mostly newer data up to March 2025. Though apparently its knowledge is "most reliable" up to January 2025, which implies there is something different about the newest stuff. Possibly it was similar for Opus 3 pretraining, and its knowledge is "most reliable" up until sometime earlier in 2023, such as April?