Should PIN ever expire?

In BW Settings (in Chrome) I have specified Vault Timeout = Custom, 96 hours, timeout action = lock. I also specified “Unlock with PIN”. The way I see it, I should be able to unlock with PIN, but every 96 hours I need to re-logon with the password. Am I wrong to expect that? It looks like in fact the PIN works indefinitely, and I never have to enter the password.

No, you have made two incorrect assumptions. First, you will not be required to re-logon after a time-out, because your Time-out Action is set to “Lock”, not “Logout”. Second, the time-out timer starts from when you last interacted with the Bitwarden browser extension, not from when you first logged in. Thus, with your current set-up, if you leave your browser extension unused and unlocked for 96 hours, it will automatically lock. You will be able to unlock with your PIN until you manually log-out, or until you restart your browser (if you enabled the option to require master password on restart).