Hearts of Iron IV asks you to win a world war, not to click every shell or propeller. Its naval and air layers are built around missions, task forces, and air wings that feed into the wider conflict. Understanding how those systems talk to each other is what turns a scattered fleet and a few squadrons into a coherent strategy.

How it works

Ships are grouped into task forces, which sit inside fleets and can be assigned missions such as patrol, strike force, convoy escort, convoy raiding, minelaying, and minesweeping. The 1.6 “Ironclad” update and the Man the Guns expansion introduced this structure alongside a ship designer that lets you fit modules to hulls rather than research whole pre-built classes. The same hull can be tuned for screening, anti-aircraft work, or long-range raiding depending on the modules you install. The expansion also added naval treaties that place dynamic limits on new designs, the ability to restrict travel through dangerous sea zones, and docking rights so allied fleets can use each other’s ports.

Patrol task forces are the eyes of the navy. They spread out across sea zones, slowly building spotting progress on enemy task forces. Spotting is not guaranteed: it advances hour by hour, and the game also rolls a random encounter chance based on that progress. When a patrol force finds a target, your strike force can be called in to fight. Without enough detection, even a large fleet can sail past an enemy without engaging.

Detection and ship design

The 2022 Naval Rebalance Designer Corner thread explained that Paradox wants spotting to be clearer and more predictable. Spotting progress now has a minimum floor, so a target will eventually be found if it stays in a zone long enough, and task-force tooltips show sub and surface detection values. Radar, fire control, and homing torpedoes improve hit chances against ships, while dual-purpose guns add anti-aircraft fire without completely giving up surface damage.

The rebalance also tightened design rules. You can no longer mix medium and light cruiser guns on the same hull, a change meant to stop ships from accidentally gaining screening protection they should not have. Speed is harder to push to extremes, and armor values were reviewed so that heavy guns, piercing, and protection matter more. The naval tech tree itself was split into a Naval tab for hulls and a Naval Support tab for equipment such as guns and shells, while dual-purpose guns now branch off the light-gun line and give both surface and anti-aircraft fire.

Air organization and missions

Air power is divided into wings of fighters, heavy fighters, tactical bombers, strategic bombers, close air support, naval bombers, and transport planes. Developer Diary 9 described the air technology tree as built around three airframe sizes: light, medium, and heavy, with carrier-capable variants unlocked as sub-technologies. That design choice means any carrier-capable plane can be assigned to a carrier, letting you decide whether your carriers carry fighters, naval bombers, or a mix.

Missions include air superiority, interception, close air support, strategic bombing, naval strike, port strike, logistics strike, and kamikaze. Air superiority is the foundation: it reduces enemy air effectiveness over a region and is often a prerequisite for successful naval invasions and ground offensives. Close air support and tactical bombers directly support land battles, while naval bombers and port strikers target ships and harbors.

Carrier air in naval combat

Carrier-based aircraft behave differently when a carrier is caught in a naval battle. According to a 2023 clarification from HOI4 game designer C0RAX, the first module slot of a carrier plane determines its default carrier mission: a fighter module makes it a carrier fighter that scrambles for air superiority, while a torpedo or bomb module makes it a carrier naval bomber that performs naval strikes. When a carrier plane is manually assigned to a mission away from the carrier, it can use any mission its modules allow.

Detection matters here too. If one side has little or no detection in the sea zone, its aircraft may not be able to fight back effectively even when they are present. This makes patrol task forces and radar coverage vital not only for finding ships, but also for giving your own carrier planes a fair fight.

How land, sea, and air interact

The three layers are not isolated. Naval strike forces can be called in to support coastal land battles, and air wings can be attached directly to armies to follow them and operate in the relevant air zone. Waking the Tiger added volunteer air wings, strategic-bomber target priorities, and an air-supply mission that drops supplies to encircled or low-infrastructure troops. Static anti-aircraft batteries in a state reduce bombing damage and shoot down attacking planes, and both fleets and air wings consume manpower for shore and maintenance crews.

Straits and canals have their own rules based on who controls the surrounding provinces, which can block fleets, restrict submarines, or allow only trade. Control of choke points therefore shapes where navies can move and where air bases matter most. Man the Guns also let players set sea-zone access to avoid or block paths, helping fleets steer clear of minefields or sub-infested waters, and added docking rights so allied navies can stage from foreign ports. Naval anti-aircraft guns now reduce incoming air damage and fire back at attackers, making escort composition matter against air-heavy opponents.

Build path

A practical naval-air build path looks something like this:

  1. Start with screens. Build destroyers for convoy escort, anti-submarine patrol, and minesweeping, plus light cruisers for heavier patrol work. Screens protect capital ships and are cheap enough to cover multiple sea zones.
  2. Add a small strike force of capital ships or heavy cruisers and keep it near a major port. Strike forces only matter if patrols can find targets, so do not build a large strike fleet before you have the detection to use it.
  3. Build naval bombers and assign them to naval strike over contested shipping lanes or sea zones where your fleets operate. Add fighters on air superiority over the same regions.
  4. If you have carriers, design carrier-capable fighters and naval bombers. Keep the carrier count modest and screen the carriers well; overstacking and positioning penalties punish fleets that are too large or unbalanced.
  5. Research dual-purpose guns, radar, fire control, and the next hull generation before refitting older ships. Refitting is cheaper than building from scratch, but it still uses dockyards.
  6. Watch fuel and manpower. The fuel system introduced in 1.6 means ships and planes need oil-derived fuel, and running dry cripples mobility and combat power. Fleets and air wings also lock up manpower, so a massive navy can drain the same pool that feeds your divisions.

Roadmap and future outlook

Confirmed changes:

  • Hearts of Iron IV launched on June 6, 2016.
  • The Waking the Tiger expansion and 1.5 “Cornflakes” update added chain of command, army attachments for air wings, strategic air priorities, air supply, volunteer air wings, and decisions.
  • The Man the Guns expansion and 1.6 “Ironclad” update shipped on February 28, 2019, adding the ship designer, task-force missions, fuel, naval mines, refitting, docking rights, and naval treaties.
  • Patch 1.3.3 improved naval AI, including having strike fleets support coastal land combat.
  • The 2022 Naval Rebalance Designer Corner is an ongoing developer-led look at tech trees, ship design, hit profiles, spotting, and fleet composition.

Rumored or unconfirmed:

  • Beyond the items Paradox has already discussed in dev diaries, the exact contents and dates of future naval or air-focused updates have not been announced. The 2017 “Future and Cornflakes” roadmap listed long-term goals such as improved naval interfaces, more player control over naval warfare, better submarine mechanics, fuel, spies, and a chain-of-command system; many of these have since appeared, but the remaining entries should be treated as development intentions rather than promises.