I can't see any evidence to suggest that 4 year terms are the most common. Briefly scanning down the list on this Wikipedia page and picking out random countries to check, 5 year terms seem to be the most common number.

America is the most obvious four year term nation, and is accompanied by Israel, but Mexico has six year terms(limited to one term per person), as does Russia(2 term limit).

The most interesting examples however, are the Commonwealth nations. With the Queen still serving as the Head of State, the terms of their Prime Ministers are "at Her Majesty's pleasure", and the PMs serve indefinitely, until parliament is dissolved for new elections of representatives. This is why New Zealand generally has 3 year 'terms', the UK has 5 years, Australia has 6 months(I kid, I kid), and Canada has four years.

But they aren't 'terms' in the same sense as the Presidency in America, because they are dependent on elections being called for the respective Houses of Representatives, which varies greatly. The United Kingdom has had several elections that were only four years apart, while having others that were five.

So the answer to your question is that it is dependent on the political system of the country. Whereas Commonwealth nations have the Queen as the head of state, others like Russia, America, and Israel, have Presidents. Commonwealth nations can't all have the Queen present, and use Prime Ministers to counter the undemocratic notion of a hereditary ruler controlling government. These PMs usually come from the parliament, as the leaders of one of the political parties. In the United States, this differs as the President is not a sitting member of the Congress. The President is elected in his own right, whereas in the Commonwealth countries, the PM is elected only if his party wins an election.

So right now, the US House of Representatives is led by Republicans, and in New Zealand or Australia, this would make Kevin McCarthy Prime Minister as the leader of that party.