Some statistics for you on beta 4:
- Average length: 14.71 days
- Median length: 14 days
- Shortest length: 8 days
- Longest length: 37 days
- # of times beta 5 was released the following week after beta 4 release: 3 times out of 14 releases (21.4% of the time)
- In other words, 78.6% of the time, beta 4 lasted 12 days or longer.
Additionally, those three times that beta 5 released in the week after beta 4, beta 4 lasted 8 days each time. Since beta 4 first came out last Wednesday, if history repeats itself, it would be released this Thursday at the earliest with the public beta coming out the next day on Friday. A Friday release seems unlikely, so either Apple could release both developer and public beta tomorrow, or Apple will go with the more probable solution: releasing it next week.
Source: Thinky Bits + math