Can a singe Dropbox'ed 1Password db be shared between 1Password v4 & v3?
See How do I use 1Password 4 in Snow Leopard or Lion?:

1Password 4 for Mac requires Mountain Lion or later.
If you are still rocking an older Mac, we’ve got you covered. We know that you store your most important information in 1Password — we do too! — so we made sure you could still use 1Password 3 for Mac on your older Macs if you need to. 1Password 4 for Mac can sync its data with 1Password 3 for Mac using Dropbox so there’s no issue using both. Access all your data, even on Macs that cannot be upgraded to Mountain Lion. Rock on!