One of the main themes of the Odyssey is the reestablishing or reasserting of an identity that is lost. How do the last four books reflect this?
In the last four books, Odysseus has arrived back in Ithaca, disguised as an old beggar by Athena. He reveals his true identity to his son, Telemachus, and makes plans to take back control of his home from the suitors and establish himself once again as king. He methodically slays the men to avoid a civil war, and finally reveals himself to his wife as her long lost husband.
Telemachus has never known his father since he left for the Trojan war shortly after his birth. His coming home reestablished his presence as a father who must teach his son the ways of a righteous king.
Penelope had nearly given up hope when Odysseus finally returned. His arrival infused her with hope, as is obvious when she constantly questions him about details of the house that only he would know. His answers assure her that he is indeed her husband; he reveals his true identity to her and she feels the ability to finally relax.
As Odysseus takes back his home and reasserts himself as king, he has come full circle as a changed man. He is older, stronger, and wiser. He has reestablished his own identity within himself, more sure than ever before in his life that his family and responsibilities to his kingdom are more precious than all the glory of war, although it was ironically that very thing that made him the man he became.