Can password vaults be shared between versions 3 and 4?
I plan to buy v4 on my iPhone 5 (in hopes the widely-reported 1P problems on IOS7 clear up). Have been very happily using v 3.8 on my Mac (Snow Leopard), with Dropbox set up to sync data. (DropBox formerly shared vault with a Blackberry; I also like the option of Web access.)
Must I upgrade my Mac to v4?
No need to upgrade to 1Password 4 on Mac. 1Password 4 for iOS can sync its data with 1Password 3 for Mac using Dropbox, so there’s no issue using both. In fact, if you're still using Snow Leopard you couldn't upgrade to version 4 (requires OS X 10.8 or later).
Hope that helps! :)