We are aware of this situation and an internal ticket has been created in order to have the app updated from our side.All you need to do is to create a ticket and the support team will be able to assist you if you haven't already done so.
I'm in the same boat, the existing Okta app is hardcoded to v3. I was able to get the necessary information from Mimecast support directly to manually create a custom SAML app and process SSO to v4. Without a guarantee of the new app from Okta being ready before the 9/6 cutoff I didn't want to be locked out. Hope this helps.