Is it true Philip of Valois and Edward III both claimed right to the Spanish throne after the death of Philip IV's last son?

Yes, both Philip of Valois and Edward III claimed the right to the Spanish throne through their descent from Philip IV of France. Philip of Valois claimed the throne through his mother, Isabella, while Edward III claimed it through his mother, Isabella, who was the sister of the last son of Philip IV. This conflict eventually led to the beginning of the Hundred Years' War.