Like previous games in the series, Hearts of Iron IV is a grand strategy wargame that focuses on World War II.

For the ground forces, the player may train, customize, and command divisions consisting of various types of infantry, tanks, and other units.

Mobilization is represented as a "policy" that the player may adjust with the proper amount of political power, an abstract "resource" that is also used to appoint new ministers and change other facets of the nation's government.

[5] Land in Hearts of Iron IV is divided into tiny regions known as provinces (also called tiles), which are grouped to form states.

Similarly, major seas and oceans (for warships) and the sky (for warplanes) are divided into different zones known as strategic regions.

[7] This diplomacy is further expanded through the addition of espionage in the expansion La Résistance, which extends gameplay in the management of occupied territories which is done differently according to player choices and ideology.

Furthermore, the operation of spy networks allows nations to steal technology, gather information on an opponent's military, and engage in other espionage efforts.

Depending on the circumstances of a nation, like their ideology, a certain level of world tension may be necessary to perform certain actions, like justifying war against another country.

[1] "Torch" "Oak" "Cornflakes" "Ironclad" "Husky" "Collie" "Barbarossa" "Avalanche" "Stella Polaris" "Bolivar" "Kaiser" On 4 April 2024, Together for Victory, Death or Dishonor, and Waking the Tiger were integrated into the base game and are no longer sold separately or considered DLC.

[2] It sold more than 200,000 units within two weeks of its launch, which made it the fastest-selling historically themed Paradox title by that time, ahead of Crusader Kings II and Europa Universalis IV.

It was the first Hearts of Iron game to reach the million mark, and the third Paradox title after, Crusader Kings II and Europa Universalis IV to do so.

[51] GameSpot gave the game a positive review, writing that "Hearts of Iron IV embodies the hard truths about all-consuming war and the international politics that guide it."

It argued that the tutorial was the only weak point, and that "for the dedicated, Hearts of Iron IV could end up being the best grand strategy game in some time.

[57] IGN wrote a positive review, describing it as "an incredibly complex World War II simulation that will require potentially hundreds of hours to master, both in-game and poring over wiki articles that read like an economics textbook", but adding that "the payoff is brilliant for those willing to put in the time to learn".

The review praised the layout, writing "thanks to an unusually striking look and clean, easily navigable interface, the biggest challenges Hearts of Iron 4 presents us with are the good kind: strategic planning, division composition, and fine-tuning economic and political policies".

The biggest problems I can point to are almost all performance-related, putting a slow, frustrating finale on what is otherwise an ingeniously detailed strategic stimulation of just about every aspect of 20th-century global warfare".

[53] A review in PC Gamer described it as a "unique, beautiful, thrilling wargame", specifically praising its frontline system and production mechanics, but also criticising the ideology system for being too bland, and finding that combat was somewhat unintuitive, writing "while I found several flaws when I stood close to the tapestry, it's important to remember that Hearts of Iron 4 exists to encompass the whole sweep of the war".

[54] Polygon praised the openness of the game, writing that "Hearts of Iron 4 goes a step further, allowing players to take control of nearly every single nation-state in the world during the same period.