In 1983, Deere introduced the 4050, 4250, 4450, 4650, and 4850. These tractors were essentially the same machines as the Iron Horses they replaced, but with significant upgrades. They offered a new 15-speed PowerShift transmission and were available with an optional mechanical front-wheel drive featuring caster action for better traction and a tighter turning radius. They also featured cosmetic upgrades, including a new light brown cab interior, instead of the black interior on previous models. These tractors were followed by the mechanically similar 55 and 60 series tractors before they were replaced by the Deere's completely redesigned 7000 and 8000 series tractors in the early 1990s.