The Mirage IV shares design features and a visual resemblance to the Mirage III fighter, featuring a tailless delta wing and a single square-topped vertical fin. However, the wing is significantly thinner to allow better high-speed performance and has a thickness/chord ratio of only 3.8% at the root and 3.2% at the tip; this wing was the thinnest built in Europe at that time and one of the thinnest in the world. While being significantly smaller than an expensive medium bomber proposal for the role, the Mirage IV was roughly three times the weight of the preceding Mirage III.
The Mirage IV is powered by two SNECMA Atar turbojets, fed by two air intakes on either side of the fuselage that had intake half-cone shock diffusers, known as souris ("mice"), which were moved forward as speed increased to trim the inlet for the shock wave angle. It can reach high supersonic speeds: the aircraft is redlined at Mach 2.2 at altitude because of airframe temperature restrictions, although it is capable of higher speeds. While broadly similar to the model used on the Mirage III, the Atar engine had a greater airflow and an elevated overspeed limit from 8,400 rpm to 8700 rpm for greater thrust during high altitude supersonic flight. While the first Mirage IV prototype was fitted with double-eyelid engine nozzles, production aircraft featured a complicated variable geometry nozzle that automatically varied in response to the descent rate and airspeed.
The aircraft has 14,000 litres (3,700 gal (US)) of internal fuel, and its engines are quite thirsty, especially when the afterburner is active. Fuel was contained within integral tanks within the wings, as well as a double-skinned section of the fuselage between and outboard of the inlet ducts, underneath the ducts and engines, and forward of the main spar of the tail fin; this provided a total internal capacity of 6,400 kilograms (14,000 lb). A refueling probe is built into the nose; aerial refuelling was often necessary in operations as the Mirage IV only had the fuel capacity, even with external drop tanks, to reach the Soviet Union's borders, thus refuelling was required to allow for a 'round trip'. In the event of nuclear war between the major powers, it was thought that there would be little point in having the fuel to return as the host air bases would have been destroyed; instead, surviving Mirage IVs would have diverted to land at bases in nearby neutral countries following the delivery of their ordnance.
The two-man crew, pilot and navigator, were seated in tandem cockpits, each housed under separate clamshell canopies. A bombing/navigation radar is housed within an oblique-facing radome underneath the fuselage between the intakes and aft of the cockpit; much of the Mirage IV's onboard avionics systems, such as the radar communications, navigational instrumentation, and bombing equipment, were produced by Thomson-CSF. Other avionics elements were provided by Dassault itself and SFENA; one of the only major subsystems not of French origin onboard was the Marconi-built AD.2300 doppler radar. Free-falling munitions could also be aimed using a ventral blister-mounted periscope from the navigator's position.
The Mirage IV has two pylons under each wing, with the inboard pylons being normally used for large drop tanks of 2,500 litre (660 gal (US)) capacity. The outer pylons typically carried ECM and chaff/flare dispenser pods to supplement the internal jamming and countermeasures systems. On later aircraft, this equipment typically included a Barax NG jammer pod under the port wing and a Boz expendables dispenser under the starboard wing. No cannon armament was ever fitted aboard the type. The early Mirage IVA had a fuselage recess under the engines which could hold a single AN-11 or AN-22 nuclear weapon of 60 kt yield. The Mirage IV can carry 12 solid-fuel rockets diagonally down below the wing flaps, for rocket-assisted take off (RATO).
From 1972 onward, 12 aircraft were also equipped to carry the CT52 reconnaissance pod in the bomb recess. These aircraft were designated Mirage IVR for reconnaissance. The CT52 was available in either BA (Basse Altitude, low-level) or HA (Haute Altitude, high-altitude) versions with three or four long-range cameras; a third configuration used an infrared line scanner. The CT52 had no digital systems, relying on older wet-film cameras. During the 1980s, a total of 18 Mirage IVs were retrofitted with a centreline pylon and associated equipment to carry and launch the nuclear ASMP stand-off missile. The Mirage IVA could theoretically carry up to six large conventional bombs at the cost of drop tanks and ECM pods, such armament was rarely fitted in practice.