Final answer:
Polish King Władysław III collaborated with Janos Hunyadi in the 1440s and led European forces against the Ottoman Empire at the Battle of Varna where he was killed.
Step-by-step explanation:
The Polish king who collaborated with Janos Hunyadi in the 1440s was Władysław III, also known as Władysław of Varna. Their collaboration was part of the efforts to defend Europe against the Ottoman Empire during that period. For instance, Władysław III led the Christian forces, which included Hungarian troops commanded by Hunyadi, against the Ottomans at the Battle of Varna in 1444.
Unfortunately, this battle ended tragically for Władysław III as he was killed in action, an event sometimes represented in historical art, such as the Ottoman miniature that depicts his death.
It's important to note that Władysław III is often confused with other historical figures due to similar names and titles, such as Casimir IV, Sigismund I, and Stefan the Great. However, none of these were the Polish king allied with Janos Hunyadi in the 1440s.