This is a bug in the QuickBooks Online product report. Every Vendor Credit will appear with two entries when you run a report under the Products and Services tab.
It'll show up like this:
Rest assured that QuickBooks does not add an additional 4 quantity to your inventory.
Vendor Credits are generally Amazon reimbursements for lost or damaged inventory, so any Vendor Credit should only subtract from your quantity on hand.
You can verify that QuickBooks Online doesn't add items with a Vendor Credit by creating a new Vendor Credit for 100 quantity of an item. You should see your inventory on hand drop by 100.
Make sure you delete the test Vendor Credit after you're done, or you may end up with a drastic inventory discrepancy!