John Key declared on The Nation that he would serve out a fourth term if re-elected. That would make him the third-longest-serving Prime Minister.
King Dick Seddon did four and a-bit terms from 1893 to 1906 — 13 years, 41 days.
Farmer Bill Massey did most of four terms, from seven months after the 1911 election to six months before the 1925 one — 12 years, 10 months. (One term was five years.)