I was just invited to the beta so I eagerly installed 1Password 4. It automatically recognized that I was sync'ing with dropbox on 1Password 3 (not running, but installed).
After starting up I had nothing in my database. It appears as if I was starting from scratch. However, when I looked at the Preferences, it showed me that I was sync'ing with dropbox and the 1Password.agilekeychain.
To figure out what might be going on, I stopped syncing and I started the dropbox sync'ing again and selected my 1Password.agilekeychain in my dropbox folder. This time it imported everything correctly and appears to be working fine.
I wonder if this is because I have some old remnants of old 1Password installs in my Dropbox folder. I had a real problem with dropbox syncing in the past. I have these files in my dropbox folder that I have never felt sure which ones were being used so I never deleted them:
1Password (this is a folder)
1Password 2011-12-10 150949.agilekeychain_zip
backup pass / 1Password.agilekeychain
Anyway, I am up and running, but I figured I would point out that the installation didn't go smoothly.