Filter products by the quantity and how to set a special price?

You can filter the products as follows. 

Please go to Dashboard - Catalog - Product and at the top in the Quantity field you can write for example "<5" and then on the filter button on the top right.

It will list all of your items with a quantity of less than 5. Check this image https://prnt.sc/vi028t

From there on, you can select each product that you like, click on the S button from the right, and put the special price on it. Once a special price is set, those items will appear in the Specials category.

Check this image https://prnt.sc/vi04s2