I have purchased SharePlus PRO before version 4.0 was released. In spite of this, SharePlus is asking me to subscribe in order to access all my content.
The steps above do not apply if you had purchased SharePlus PRO via Apple's VPP program. If this is your case, or the above steps didn't work, please send a support request from within the SharePlus app (Settings > Feedback & Support > Get Support).
I hope this helps.
Here's a photo walk-through of the process:
Take care,