Possibly a point of pedantry I know, but bear with me.
The 7 Year War was fought in the second half of the 18th century. I involved not only basically all the countries in Europe but also spread to 4 other continents. It saw the French being defeated in North America and determined that the New World would be speaking English, not French. Taxes raised by the British government to help pay for the war were a major factor in triggering the American War of Independence. In India, rival French and British trading companies allied themselves with local kingdoms and warlords. In defeating the French and their allies, the British East India Company gained control over vast swathes of territory, setting them on the road to Empire in India. The war also saw Prussia emerge as a great power. By any reasonable definition, this was a world war.
The Napoleonic Wars saw a French army make it to Moscow. They lead to Britain occupying the Cape and Southern Africa and to further territorial gains in India. British troops burnt down Washington and the nascent US navy took on the British in the Atlantic. Fighting in South America saw Spain’s colonies in the region begin to break away from their mother country.
If we accept that both conflicts were world wars then that makes the Second World War really at least the Forth World War.