World's longest ruling head of government is from:

1. The question asks about the country with the world's longest ruling head of government as of 2004. 2. Fidel Castro of Cuba assumed power in 1959 and served continuously as Prime Minister and later as President until he stepped down due to health reasons in 2008 (officially handing over presidency in 2008). By 2004, he had been in power for about 45 years, making him one of the world's longest-serving heads of government at that time. 3. Leaders in Switzerland have collective leadership with rotating presidency, so no single long-ruling head. Robert Mugabe of Zimbabwe came to power in 1980, ruling for a shorter duration than Castro by 2004. Leaders in New Zealand have standard democratic terms. 4. Therefore, Fidel Castro of Cuba was arguably the longest-ruling head of government among the plausible options in 2004.