When opening your Sparkle V3 project file in the new Sparkle 4 you need to save it. What you have done is convert your old Sparkle V3 project file across to the new Sparkle 4.
Everything after that will work as normal.
Enjoy!!! 
Correct, with the only difference, that once you have converted the project, it cannot be used with Sparkle 3 any more.
This is a fact, since Sparkle 4 is not yet available in the App Store, so I downloaded the trial version from the homepage (and use only the trial) - and I cannot switch the applications (at least today - until I get the App from the App Store).
Any suggestions on transferring the FTP publishing information to the new version without having to reenter everything? I duplicated the last file, then opened it in v4, but the FTP info doesn’t show. They show in the Sparkle App preferences (v4 & v3) with “Remove” as the only option. They remain in the v3 file but not the v4.
Suggestions? Thanks!
Unfortunately Apple seems to be particularly slow at updating their content delivery. We even gave it some lead time, but even though Sparkle 4 is published on the AppStore from the point of view of our dashboard, it appears to still be largely unavailable worldwide.
This is what we see:
any yes the update is not available on my appstore either…
We’ll survive one more day 
You can try going to the keychain and expressly granting permission to Sparkle 4 to access the keychain items.
All good. Version 4 is available from the App Store - at least in Germany
Apparently the appstore is still acting up on pricing, showing random prices within Sparkle (and the correct price once you click through to upgrade). Mysterious.