OK, here's what I did:
System version: 17A306f (Developer Beta 3 = Public Beta 2, but the same had worked for PB1 before).
The system would not update to DB4/PB3, it would only restart without installing a new version.
- I restarted macOS, holding "D" key while restarting.
- "macOS Utilities" window appeared.
- I selected "Reinstall macOS" (2nd option from the top) and clicked "Continue".
- "macOS High Sierra Beta" window appeared, I clicked "Continue" again.
- The license was displayed, I clicked "Agree" and then "Agree" again in the disclaimer window.
- I selected the installation disk in the next window and clicked install.
- The computer restarted, I entered my password and the actual installation began with black screen and installation time (about half an hour) in white.
After logging in again, the system version is 17A315i now (i.e. DB4/PB3).
Hope this helps!