In 4Seller, if the orders are "Out of Stock", you can refer below 2 methods turn to "To-Ship"Status.
Method One: Click "Out of Stock"
If it's a proprietary warehouse, you'll be redirected to the inventory addition page to add inventory, click "Save".
If it's a third-party warehouse, you'll be redirected to the SImple product page to view inventory, adding inventory requires manually adding it from the third-party warehouse.
Method two: Change Warehouse
If the stock sku not just in one warehouse, you can switch to a warehouse with inventory.