In my case I found nothing and retrieved all my outlook items and deleted the old OST the same day. The step 7 can be done after few days or weeks if you believe you lost something. Launch Outlook, the OST file would be re-created and all email from the mailbox downloaded again
The only way I found was to delete the local OST file (I put in in the bin until I was sure I would not loose anything). I tried all the other answers solution and even finding a way to disable caching, the counter was 0 but when I re-enabled it the counter came back to 4 billions. When looking the counter it shows the 4 billion unread message and when looking into the footer I notice it is negative. I had similar issue with a unread email count of more than 4 billions.
