Answer number 1:

In the centre of the composition Leonardo da Vinci as Plato (of whose philosophy it has been said: All that came after are merely footnotes)

Raffaello's Leonardo/Plato.

A self portrait by Leonardo

Answer number 2:

The figure of Michelangelo Buonarotti as Heraclitus was not in the original cartoon & is said to have been added after Raffaello snuck into the Sistine chapel one night to see Michelangelo's progress on the ceiling. He was so awed by the sculptor's work he added the figure to the foreground in homage to the maestro.

Raffaello's Michelangelo/Heraclitus

A drawn portrait in profile of Michelangelo.

Answer number 3:

Raffaello Sanzio

He pictures himself modestly in the background though he chooses the legendary painter Apelles (4thC BC) as his avatar. Apelles was Alexander Magno's 'court' painter, he was remembered by Pliny the Elder as unsurpassed in talent before or after. Homage or arrogance?!

Raffaello's self-portrait as Apelles is easy to spot because it looks directly at the viewer, this is common to most self-portraits because they are painted looking into a mirror.

Attributed by some to Raffaello as a
self-portrait.

Answer number 4:

Bramante D'Urbino, The greatest architect of the high Rennaisance as Euclid, arguably the greatest mathematician of ancient Greece.