Article 69 of Hong Kong Basic Law (香港基本法第六十九條) sets the term of the Legislative Council of Hong Kong (LegCo).
The article states the term of office of the Legislative Council of the Hong Kong Special Administrative Region shall be four years, except the first term which shall be two years.
This defines the reason for the LegCo election irregularity - 1998, 2000, and then 2004. The First Legislative Council was in office two years and three months (1 July 1998 to 30 September 2000) and subsequent Councils have served terms numbering precisely four years. Council members - 30 each elected from geographical and functional constituencies, hold four year terms.
Each term is divided into four sessions each running from October 1 to September 30.