What is chapter 4 about in the book City of Ember?

Lina has been a messenger for several weeks when she notices that Granny is acting strangely.  She is stripping the cushions from the couch, searching for something which "is lost", but she doesn't know exactly what it is that is missing.  Most significantly, Granny is so wrapped up in her nebulous search that she has forgotten the baby, having left her downstairs alone in the shop. 

Lina is given a message to deliver to Clary, the greenhouse manager.  The greenhouse is where the food for Ember's citizens is produced; it is located at the farthest edge of the city near the trash heaps.  Recently, an official job called "trash sifter" has been created.  Trash sifters search through the trash looking for anything that might be useful, leading Lina to wonder if it is true that Ember is running out of everything.

Lina delivers the message to Clary.  The message is a request for extra crates of cabbages and potatoes from Arbin Swinn, who runs the Vegetable Market.  Clary responds that she can provide the cabbages, but not the potatoes, as the potatoes have been stricken with a new disease.  Unless a cure is found for the disease, there will soon be no more potatoes in Ember.

Just before Lina leaves the greenhouse, she hears the sound of wailing.  Clary urges Lina to go, but Lina witnesses Clary encountering Sadge Merrall, a clerk in the Supply Depot.  Sadge has apparently made an attempt to penetrate the Unknown Regions, the completely lightless area surrounding all of Ember.  People have always wondered if anything lies beyond their city, but no one has been able to get very far into the Unknown Regions because a portable method of lighting has not been invented, and the Unknown is characterized by complete darkness. 

As supplies of one sort and another begin to run short in Ember, more and more people are looking to the Unknown Regions, in hopes that there might be something there that can help them.  Clary and Lina talk about questions that have troubled both of them, and many others, for a long time - the mystery of life, and the origin of life.  They wonder if the answer to their questions might indeed lie in the Unknown Regions (Chapter 4).