The International DuraStar line, known as the 4000 series prior to 2008, is a line of medium-duty trucks produced by Navistar International from 2001 until 2018. Introduced as the successor to the International 4000 series of 1989–2001, the 4000 series was renamed the DuraStar in 2008. Developed as a Class 6-7 product range, the 4000/DuraStar was slotted below the 8000/TranStar regional-haul semitractor, with the Class 5 International TerraStar (2010–2015) serving as the smallest International conventional-cab product range.

The most distinctive features of the DuraStar are the "crescent shape" headlights and a distinctive "black spot" on the left side of the cab. Produced as both a semitractor and a straight/rigid truck, the 4000/DuraStar has been used in a wide variety of applications, including emergency vehicles, towing, flatbed trucks, and cargo box trucks. For bus use, the chassis is used in both cowled-chassis and cutaway-cab configurations for school bus and commercial applications.

The DuraStar was replaced by the International MV Series in 2018.

In early 2001, Navistar entered into a 50/50 joint venture to produce commercial vehicles with Ford Motor Company, officially named Blue Diamond Truck, LLC. Under the agreement, Navistar assembled medium-duty trucks for both companies in its facility in General Escobedo, Mexico.

Though externally different, both Ford and Navistar model lines would share a common chassis; the medium-duty Ford F-Series (which had already entered production in 2000 at General Escobedo) shared its cab with the F-Series Super Duty line (F-250 through F-550). Though smaller F-Series vehicles used Navistar-supplied diesel engines, medium-duty Fords used engines supplied by Cummins and Caterpillar.

Serving as the debut line of the NGV (Next-Generation Vehicle) cab structure, the new International 4000 series sourced its engines from within the company, carrying over the T444E V8 and DT466E I6 diesel (later VT365, DT466, and MaxxForce engines) from the previous 4000 series (sharing other commercial-grade drivetrain components with Ford and other medium-duty vehicles).

In 2015, Ford ended its stake in the joint venture; as part of a redesign, production of medium-duty trucks was relocated from Mexico to Ohio (replacing the E-Series van), with Navistar continuing production at General Escobedo.

In February 2001, Navistar released its all-new 4000 series, dubbed as "High Performance Trucks". Developed in the Blue Diamond Truck joint venture with Ford, the 4000 series marked the first completely new truck line from International since the 1979 S series. Sharing its chassis with the medium-duty Ford F-Series (introduced in 2000), the International 4300/4400 retained the DT466 and DT530 diesels of their predecessors. For versions equipped with automatic transmissions, electronics for the engine and transmissions were retuned to optimize throttle and shifting response, to increase both performance and fuel economy. To increase forward visibility, the size of the windshield was increased by over 60%, with additional attention paid to improvements in ventilation.