In Hearts of Iron IV, units are the basic instrument through which the player wages war. As such, they are extremely important.
- Main article: Land units
Land units in Hearts of Iron IV are represented on the map by divisions. Divisions subdivide into regiments and then into battalions, and can be customized via the Division Designer. There is a maximum of five battalions in a regiment and five regiments in a division. Such regiments are in some countries referred to as "brigades". Also, up to five support companies can be attached to a division. Land units can be split into four groups: leg infantry, mobile infantry (e.g. cavalry and motorised infantry), tanks and support units (artillery, anti-tank guns, etc).
- Main article: Naval units
Naval units are represented by single ships, and - unlike land and air units - are produced with shipyards. Naval units are assigned to strategic areas but operate in individual sea zones. Ships are divided into destroyers, cruisers (both heavy and light), battlecruisers, battleships (both normal and super-heavy), carriers, submarines, transports, and convoys. The latter two have no direct on-map presence. As in Hearts of Iron 3, more obscure ship classes (e.g. pre-dreadnought battleships or armoured cruisers) have been abstracted into the previously listed types.
- Main article: Air units
Unlike in previous Hearts of Iron games, the atomic unit for air units is not air wings or squadrons. Instead, each air unit represents an individual aircraft. Aircraft are deployed into strategic regions, and to obtain air superiority opposing air units in a strategic region must be overwhelmed. Aircraft divide into light, medium, and heavy airframes filling multiple roles, and can be customized with experience.