The first three parts of this essay can be read here, here, and here.
In one battle, Alexander had changed the political landscape of the Mediterranean and Asia (what we now think of as Asia Minor and the Middle East) forever. Functionally, he now controlled all of Greece and Asia Minor and became master of an unimaginable hoard of slaves and treasure seized from the Persians at Damascus. Despite his upbringing as a prince of a nation flush with military success, the spoils won from the Battle of Issus were enough to cause Alexander to remark, “This, it seems, is royalty” .
Plutarch uses the aftermath of the battle as another opportunity to rhapsodize on Alexander’s benevolence, detailing a half dozen or more ethical situations that arose from his success and how he, invariably, resolved them in a manner more generous than might be expected. Of greatest importance to history, as it had yet to unfold, was Alexander’s treatment of Darius’s mother, wife, and two daughters. Upon learning that they had been taken among the Persians at Damascus, Plutarch writes that:
Alexander sent Leonnatus to them, to let them know that Darius was not dead, and that they need not fear any harm from Alexander, who made war upon him only for dominion; they should themselves be provided with everything they had been used to receive from Darius. 
Despite the temptation to immediately pursue Darius and deliver the killing blow while he was still in retreat, Alexander decided instead to bring the eastern coast of the Mediterranean under his authority, whether by diplomacy or force. Some, like the peoples of Cyprus, surrendered without a fight. As a result, Alexander marched unopposed as far as modern day Lebanon before reaching the well-defended island city of Tyre which refused to submit to him.
With no fleet of his own to attack the island, Alexander first blockaded Tyre from land reinforcements and then set about building a land bridge to Tyre across which he could move his large siege engines and armies. Though it took seven months and the lives of hundreds of his men, Alexander eventually reached the city and overcame its defenses. He ordered the destruction of large portions of the city and sold most of its inhabitants into slavery as punishment for refusing his authority.
As with Thebes before it, the destruction of Tyre sent a chilling message to the other vassal nations that found themselves suddenly far stranded from their Persian rulers: Submit or be destroyed utterly. While the historical record tells us that the Siege of Gaza a few months later was an equally bloody ordeal, Plutarch chooses to skim over it for reasons that remain his own, deciding to fix instead on the strange manner in which Alexander was seriously wounded (his version includes a bird, a dirt clod, and an unruly siege engine). Regardless, by 332 BCE, Alexander had subdued the entirety of the eastern Mediterranean coast and, without pause, turned his attention immediately towards Egypt.
Unlike the coastal cities that held out against Alexander (who by and large fought for their own independence rather than out of a tortured sense of loyalty to Darius), Egypt was only too happy to welcome Alexander as their ruler. Plutarch’s first mention of the Egyptians announces their capitulation as an established fact, noting that, “when he was master of Egypt, designing to settle a colony of Grecians there, he resolved to build a large and populous city and give it his own name” . After laying out the plans for Alexandria, Alexander set out on a dangerous journey to consult the Oracle at Ammon. While Plutarch again offers little speculation on his motivation, one can presume that his intentions were at least two-fold. First, in order to legitimize (if not secure) his sovereignty over Egypt, he would have to be anointed to some extent by its religious leaders. Second, the Oracle at Ammon was as revered as that at Delphi and Alexander, it seems, had a few questions yet that he believed only a god could answer.
Having passed through the wilderness, they came to the place where the high priest, at the first salutation, bade Alexander welcome from his father Ammon. And being asked by him whether any of his father’s murderers had escaped punishment, he charged him to speak with more respect, since his was not a mortal father. Then, Alexander, changing his expression, desired to know if any of those who murdered Phillip were yet unpunished, and further concerning dominion, whether the empire of the world was reserved for him? This, the god answered, he should obtain and that Phillip’s death was fully revenged, which gave him so much satisfaction that he made splendid offerings to Jupiter, and gave the priests very rich presents. 
Every historian who approaches Alexander as a subject must wrestle at some point with the question of whether or not he believed that he was born of divinity and Plutarch is no exception. His methodology gives him some distance from the topic as he presents evidence for and against the idea throughout the whole of this biography. It is, nonetheless, tempting to look at this moment in Ammon where Alexander is informed in no uncertain terms that he is the son of the greatest god (for the priests, Ammon; for Alexander, Zeus; and for Plutarch, Jupiter) and wonder if this is when he began to believe his own hype. His success as a general was without question as he had accomplished in three years what the Greek city-states, in hundreds of years, could not. Claiming that one was a distant descendent of a god (as both Phillip and Olympias had) was the privilege of the nobility. To insist that one was the direct offspring of Zeus was an exponentially more audacious claim and yet that, whether by his own design or by those who venerated him in the chaos that reigned after his death, is the myth that survived him.
While in Egypt, Alexander received a letter from Darius outlining terms for a cease-fire. In exchange for the cessation of hostilities between them and the safe return of the hostages, Darius was willing to cede all lands west of the Euphrates to Alexander, along with a ridiculous amount of treasure and the hand of one of his daughters in marriage to Alexander to bind the agreement. Alexander sent word back that unless Darius came before him and yielded his authority, he would come and take it by force.
Soon after, Darius’ wife, Statira, died in childbirth. Plutarch explicitly states on numerous occasions that Alexander did not force himself upon her or any of Darius’s family during their captivity. The timing however of this particular death (some two years after Darius’ defeat at Issues) suggests that someone was keeping her company in the aftermath and, given the lifestyle that Alexander lavished upon Statira and the others, it makes more sense that the child would have been his as, by the rules of war, she was part of his personal spoils. With peace denied and new indignities having been suffered by Darius, there was to be nothing to stand in the way of a fresh engagement between these two empires.