He did all this long after Alexander IV's death and after he had declared himself a king (since Alexander's family had been exterminated) in order to secure the succession of his family. The arch-royalist Ptolemy did the same, despite having ruled in the name of Alexander IV, erected statues of him, issued decrees in his name etc etc.
Nope, while Seleukos was satrap Babylonian documents were dated by the regnal years of Alexander IV but as soon as Antigonos took over in 315 the dating formula changes to the years of his ‘generalship’, until he in his turn was ousted in 311 and Seleukos re-instated dating by Alexander IV until his assumption of the diadem. The same pattern is seen on the Idumaean ostraca, from which area the Antigonids were forced in312; all well before the murder of Alexander IV. ‘Facts, facts, facts!’
Diodoros gives us the story of Herakles in Book XX
20 1 Meanwhile Polyperchon, who was biding his time in the Peloponnesus, and who was nursing grievances against Cassander and had long craved the leadership of the Macedonians, summoned from Pergamon Barsinê's son Heracles, who was the son of Alexander but was being reared in Pergamon, being about seventeen years of age. 2 Moreover, Polyperchon, sending to his own friends in many places and to those who were at odds with Cassander, kept urging them to restore the youth to his ancestral throne. 3 He also wrote to the Federal League of the Aetolians, begging them to grant a safe conduct and to join forces with him and promising to repay the favour many times over if they would aid in placing the youth on his ancestral throne. Since the affair proceeded as he wished, the Aetolians being in hearty agreement and many others hurrying to aid in the restoration of the king, in all there were assembled more than twenty thousand infantry and at least one thousand horsemen. 4 Meanwhile Polyperchon, intent on the preparations for the war, was gathering money; and sending to those Macedonians who were friendly, he kept urging them to join in the undertaking.
28 1 Meanwhile Polyperchon, who had collected a strong army, brought back to his father's kingdom Heracles, the son of Alexander and Barsinê; but when he was in camp at the place called Stymphaeum, Cassander arrived with his army. As the camps were not far distant from each other and the Macedonians regarded the restoration of the king without disfavour, Cassander, since he feared lest the Macedonians, being by nature prone to change sides easily, should sometime desert to Heracles, sent an embassy to Polyperchon. 2 As for the king, Cassander tried to show Polyperchon that if the restoration should take place he would do what was ordered by others; but, he said, if Polyperchon joined with him and slew the stripling, he would at once recover what had formerly been granted him throughout Macedonia, and then, after receiving an army, he would be appointed general in the Peloponnesus and would be partner in everything in Cassander's realm, being honoured above all. Finally he won Polyperchon over by many great promises, made a secret compact with him, and induced him to murder the king. 3 When Polyperchon had slain the youth and was openly co-operating with Cassander, he recovered the grants in Macedonia and also, according to the agreement, received four thousand Macedonian foot-soldiers and five hundred Thessalian horse.4 Enrolling also those of the others who wished, he attempted to lead them through Boeotia into the Peloponnesus; but, when he was prevented by Boeotians and Peloponnesians, he turned aside, advanced into Locris, and there passed the winter.
This reads as quite a serious attempt to install Herakles, but if he was so close to regaining power why would Polyperchon swap for 4,500 troops and a Peloponnesian sinecure? If the Macedonians were so keen to see the boy king why do they acquiesce in his murder without demur? Why if Polyperchon has brought the pretender to ‘his father’s kingdom’ does he halt and camp in Epeiros? Never part of Alexander’s Empire.
Why repeat the tropes from the confrontation with Olympias; Macedonian fickleness and Kassandros’ uncertainty of their loyalty? Considering these factors it reads more like an exaggeration of a failed Antigonid ploy (Antigonos held Pergamon, his complicity can hardly be doubted).
Kassandros was much wiser than Mortimer who demonstrates that misjudgement in treating a future overlord proves invariably fatal.
When you think about, it free-choice is the only possible option.