If Apple Card is close to launching, they will release 12.4 public before 13.0 beta 4. That way, iOS 13 users can update to beta 4 and enable Apple Card. If they release beta 4 before 12.4 final, it won’t have Apple Card support and we will have to wait until beta 5. Look back on past articles on Apple Music. iOS 9 beta 3 enabled Apple Music for iOS 9 beta users, and was released a few days later than 8.4 final.
That being said, with a big release like Apple Card, Apple will announce its release (typically in their newsroom press release page) a few hours before the release of the software update. You can follow this channel on Apple News and setup braking news notifications whenever they release something to the press.
I could be wrong, but I see the imminent release of Apple Card playing a huge role in when we will get seeded with the next beta. There was an instance where we were seeded a beta and saw a public release the same day (iOS 6.0.1 and iOS 6.1 beta 1 in November 2012).