In Schedule C (Car & Truck Expenses) worksheet, what does "Less than 5 at a time must be entered" mean?

You should "Close-out" the business (Schedule C) if the business will no longer be reported. 

At that time, go to your business assets, the truck will be listed. 

Report that the truck was "Converted to Personal use".

Keep a record of the depreciation taken on the truck as this MIGHT be needed when you eventually sell the vehicle.