Chapter 7 depicts four visionary beasts: a lion, a bear, a leopard and a terrifying beast. However, those have to be seen as having their counterparts in the vision of the statue, and also with the vision in chapter 8. All the information from those other chapters have to be collated to get a clear picture.
Babylon is stated to be the head of gold on the image, its chest and arms of silver equaling the ram in ch. 8 (which is Medo-Persia).
The belly and thighs of bronze equal the goat in ch. 8 (which is Greece, including the Ptolomies and Selucids, and the later Maccabees and Hasmoneans).
Then the legs of iron equal the terrifying beast, which is Rome.
This is taken from the NIV Study Bible 1987 edition, with a helpful diagram and detailed explanations with dates (page 1289). Another Study Bible (the NLT) seem to be in agreement, and the scholars who produced them are qualified in biblical hermeneutics.
However, it is the angel who then gave Daniel the understanding of what those visions means who is The Authority. He stated in chapter 7, verse 17 that the four beasts represent four kingdoms that were to arise from the earth. Details were given about the fourth beast, but it is the leopard in ch. 7 that fits the bill for what happened after Alexander the Great's sudden and early death; not the other three beasts. Here is how the NIV Study notes explain this, after describing how it had four wings on its back, and four heads (7:6):
"The leopard with four wings represents the speedy conquests of
Alexander the Great (334-340 B.C.) and the four heads correspond to
the four main divisions into which his empire fell after his untimely
death in 323:
Macedon and Greece (under Antipater and Cassander), Thrace and Asia
Minor (under Lysimachus), Syria (under Seleucus I), Palestine and
Egypt (under Ptolomy I).
Recall how chapter 8 has to go alongside chapter 7 to get all the details? This confirms the interpretation just given. The study notes explain:
"8:3 - The ram represents the Medo-Persian empire (v. 20). The longer
of his two horns reflects the predominant position of Persia. Vs. 5 -
The rapidly charging goat is Greece, and the prominent horn is
Alexander the Great, 'the first king' (v. 21). 8:7 - Shattering his
two horns: Greece crushes Medo-Persia. 8:8 - His large horn was broken
off. The death of Alexander the Great at the height of his power (323
B.C.). Four prominent horns - four heads (7:4-7)."
It then goes on to detail the other horn emerging from one of the four horns belonging to the third kingdom (Greece). The horn that started small is Antiochus IV Epiphanes.
So, to answer the question: it is not the four beast in chapter 7 that represent the Diadochi kingdoms. It is the leopard with four heads (which is but one of the four beasts).