In 1973, Deere introduced its new 'Sound Idea' tractors, the 4030, 4230, 4430, and 4630. While these tractors were mechanically similar to the New Generation tractors they replaced, and the 4230, 4430, and 4630 used a 404-cubic-inch displacement engine like the 4020, they featured redesigned sheet metal and most importantly they were available with an optional completely integrated operator's cab that John Deere called the Sound Gard body. This insulated cab that included a roll-over protective structure had a distinctive rounded windshield and came equipped with heat and air conditioning, as well as speakers for an optional radio. An 8-track tape player was also available as an option. The 5020 was replaced by the very similar 6030 and continued in production with New Generation styling until 1977 when the 30 Series tractors were replaced by Deere's 'Iron Horses' series that included the 90-hp 4040, 110-hp 4240, 130-hp 4440, 150-HP 4640, and 180-hp 4840. The 4240, 4440, 4640, and 4840 featured a new 466-cubic-inch displacement engine, and improvements to the cab including an optional hydraulic seat for a smoother ride. The Sound Gard body and Power Shift transmission were standard equipment on the 4840.