Is The iPhone App A Separate Purchase? Do I Need To Repurchase The App For My Other iPad?

No. If you're logged in with the same Apple ID on each of your devices, you'll see iAnnotate 4 available for download in the purchased section of your App Store.

 

To access this section, open the App Store --> tap your account icon in the top right ---> tap Purchased --> navigate to or search for  iAnnotate