When do I stop filing schedule F (which shows depreciation) after selling a farm (along with the principal home)?

There should be no Sch F or any other worksheets related to the Farm if the farm is gone.

When the farm was sold 4 years ago, each asset on the depreciation schedule should have been accounted for at that time so nothing was left.

You need to delete the deprecation schedule and all the accompanying worksheets and the Sch F as it is no longer needed.