What different ways do the clones have of dealing with fourth donation? What do you think would be the best way to deal with it? What deep fear do they typically have?

The donors in Never Let Me Go know that they will most likely die after their third or fourth donation, but they have been conditioned since early childhood to accept this fate as natural. Even the vocabulary they adopt has words that serve as euphemisms to minimize the reality and injustice of their fate; for example, they use the verb "complete" instead of "die", conceiving of the final donation as the ultimate fulfillment of their purpose in the world.
The clones' specialized language for obfuscating their unjust reality works hand-in-hand with their deep insecurity about being less than human. For example, Ruth and her friends venture out of town to find her "possible," fixated on a kind of Platonic ideal of Ruth rather than the reality of Ruth and the urgency of her suffering. This fear and derealization functions very well to shepherd them along the very path that their society requires them to take, and prevents them from conceiving of a different path or a true validation of their own selfhood.