You just need to install Heroes 4 and then sequentially install all 3 parts:
1. Original,
2. The Coming Storm,
3. Whirlwinds of War.
Posts from Official patches and mods for Heroes 4 by Equlibris.
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When should I install the patches? And when will Equilibrium be released?
Install Equilibrium on top of Warlords. Read the instructions first. It's all straightforward and easy to understand.
The Equilibrium website mentioned new buildings and spells – how can I check if I have everything for a complete game experience?

Sorry if this is off-topic.
I have a question. I have a licensed copy of Heroes 4 and the expansion, Heroes 4: The Gathering Storm. What do I need to do to play online?
I downloaded the patches, but they gave me an error.
Install Equilibris, and you'll be happy. ;)
By the way, before asking such a question, you should have used the forum's search function – it's a great tool. Even in this thread, there are many posts about your problem.
Думающий поймет. Действующий совершит. Дорогу осилит идущий ©
Whirlwinds of War is the latest version. If you have it installed on top of the original Heroes without Gathering Storm, you just need to uninstall Heroes 4 and then install all three parts in sequence: 1. Original, 2. Gathering Storm, 3. Whirlwinds of War.
However, I disagree with this – Gathering Storm is not needed. I've been playing Heroes 4 for several years, and I've never had Gathering Storm installed. I always install the base game and then Whirlwinds of War (now also Equilibris) on top of it. I've done this at least a dozen times, and it always works fine.
Думающий поймет. Действующий совершит. Дорогу осилит идущий ©
by the way, a question not related to the topic, in my opinion
so, you need to reinstall the game
and then immediately install the patch
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