After the death of Ptolemy III, he was succeeded by his son Ptolemy IV. He was about 24 years when he ascended the throne. Philopator was lazy and self-indulgent and he surrounded himself by some notorious friends. Being fat, he led a promiscuous life. He took Agathacoles, who was known for his bad manners and behavior, as his best friend and his sister Agathacolia as his mistress, so the King was under their control. Sisobius was the Finance Minister during the reign of his father and he continued at the same post during the reign of Ptolemy IV.
It was Sisobius who facilitated this style of life for Ptolemy IV. His mother Queen Bernice II didn’t like her son's behavior and she decided to replace him by his younger brother who was called Magas. When Sisobius knew of the intention of Queen Bernice II, he informed Ptolemy IV. Consequently, Ptolemy IV poisoned his mother Queen Bernice II and killed his brother Magas while having a bath. He continued the bloody action by killing his uncle, Lysimachus, and continued to live his promiscuous life. When the news of the bad behavior of Ptolemy IV spread outside Egypt, the Seleucus King, Antiochus III, decided to invade Egypt.
When King Ptolemy IV knew about the intention of Antiochus III, he decided to prepare an army. However, the Ptolemaic army was in a bad condition suffering from the lack of soldiers and weapons. So Sisobius who became the advisor of the King, decided to negotiate with Antiochus III. The main aim of the negotiations was not to reach a solution but to have more time to prepare the Ptolemaic army. So, Sisobius sent some officers to Greece and Macedonia to recruit some mercenaries but this attempt failed and met with little success. So they were forced to recruit Egyptians in the Ptolemaic army for the first time in the Ptolemaic history. The Egyptians were trained in a camp outside Alexandria and they were soldiers in the phalanx under the leadership of some Greek officers. When the Ptolemaic army was almost ready, Antiochus III discovered that the negotiation led to no where, so he decides to invade Egypt. Ptolemy IV led the Ptolemaic army and they headed to the Sinai then to Raphia where the two armies met each other in 217 BC. The Ptolemaic army consisted of 50000 soldiers, most of them were Egyptians while the Seleucus army consisted of 60000 soldiers and most of them were Greeks. Each of the two armies consisted of two parts: the phalanx in the middle and the wings on the sides.
The Seleucus army had Asian elephants while the Ptolemaic army had African elephants which were smaller than the Asian ones. All this led to the superiority of the Seleucus army since all the power was in its favor The battle started and the wings of each army started to fight. Shortly, the wings of the Ptolemaic army were defeated and they escaped from the battlefield and the Seleucus army took the spoils of its foe then the battle started again with the phalanx. After a great battle, known as the Battle of Raphia, the Ptolemaic army achieved a very great victory over the Seleucus army in 217 BC and the Egyptian almost wiped them out. This battle is considered the most victorious battle during the Ptolemaic Period under this notorious king (Ptolemy IV) and this victory was mainly due to the army consisted of Egyptians as well as the Egyptian phalanx.
After this battle, the Egyptians felt that they are no less than the Greeks and when they returned from the battle, they began to revolt against the Ptolemaic rule asking for their rights. The first and biggest revolt by the Egyptians took place at Thebes in 216 BC but Ptolemy IV used excessive force to put down this revolt. However, this wasn’t the end of the Egyptian revolt and these revolts never stop in the following years. As a consequence, Egyptians began to reach high positions in the army and the administration.
Before year 217 BC or before the Battle of Raphia, the Ptolemaic Kingdom enjoyed a time of power, strength and prosperity but after year 217 BC, it started a time of weakness and decline. Nevertheless, as for the Egyptians, they began to reach high positions. But unfortunately when the Romans came they deprived the Egyptians of these high positions once again. After this battle, Ptolemy IV continued to lead his promiscuous life and married to his sister Arisnoe III. Some scholars suggested that he married his sister before the Battle of Raphia. He married his sister after his mistress (Agathacolia) failed to give him any child. After a seven-year marriage, Arisnoe III gave him a son called Ptolemy V. At the age of 41, Ptolemy IV died due to his fatness and his promiscuous life leaving his wife Arisnoe III on the throne of Egypt and his son was about 6 or 7 years old.
You can get more information here.