iPhone 5S and iPad 4 I purchased 20 GB extra of storage for my iPad and iPhone on iCloud. But my devices say they are still full. How do I get the storage to show up so I can download an update, apps etc.?
You bought extra cloud storage, not extra phone/pad memory [which is not possible].
The two are utterly unrelated. 

You need to transfer some data to a computer, or delete it, to free up extra space on the devices.