The Ford Thunderbird became a 4-passenger vehicle starting in the year 1958. The first generation of the Ford Thunderbird, produced from 1955 to 1957, was a 2-passenger convertible. However, with the introduction of the second generation in 1958, the Thunderbird was redesigned as a larger, more luxurious model and offered seating for four people. This change allowed the Thunderbird to cater to a broader market and appeal to those looking for a stylish and comfortable 4-passenger vehicle.