I'm running 1P3 on three Macs (10.8), two PC's (Win7), and two iOS7 devices. I use Safari/Firefox/Chrome on Mac and Chrome/Firefox on Windows. Sync everywhere is via Dropbox.
Other than the issue with Chrome extension syncing already discussed, are there any other known gotchas? Are there new fields in 1P4 that I should not expect to see on Windows until 1P4/Windows ships? I assume new stuff like 1P Mini, multiple vaults, and sharing vaults are not going to work cross-platform yet but I'm concerned about existing features that will stop working.
You should check the Known Issues topic to find out everything.
One thing to keep in mind is that 1Password 4 doesn't yet do full import/export and print yet but that'll be fixed in a future update..
You may see some of your items being converted to Logins that when it is imported into 1Password 4, such as pre-2010 email items incorrectly being converted to Logins instead of remaining as email items, Account items converted to Logins (Account is no longer a category in 1Password 4), and stuff like that.
However, you shouldn't lose any data, just that data would be located in different places and if you edit those items, they'd be synced back to 1Password 3 for Mac/Windows as such.
So, if you edit a Login item that is an Account item in 1Password 3 for Mac or Windows app, that will sync the item back as an Login item instead of remaining as Account item in the older apps.
If you customize an item with icon/custom fields and so on in 1Password 4, editing the same item in the older 1Password apps may result in losing some of those customizations when it is sync'ed back to 1Password 4. That's because those apps don't write those custom data.