Well as FilePlanet is long gone, and second link for download from official site is dead, I spent some time searching my hard drive for installation as it seems it's not available anywhere in universe for download.
So here it is, in case anyone wants it, uploaded to private server:
Download link
If you can't get it running from installer above try with this package, cobbed together by gammaleak.
Oh, and for those wondering it's balance mod mod for Heroes IV. If you never played with it definitely do.

Enjoy!

I agree, it makes the Heroes IV experience better - already, I have to add. Too bad that further development of this project has become so slow; the further additions the Equilibris team had planned would serve H4 well.

Thanks for the download link :)

what does it change?
does it remove some unfair spells?
does it fix the movement of slow units?
and does it work online?

This site might answer some of your questions:

http://equilibris.celestialheavens.com/eng/changes.html

Says it is not a valid Win32 app..?

I can't get this to work even after tracking down various files. The program just doesn't start. Anyone had luck making it work on Windows 7?

This post kindled my interest in trying out Equilibris, which I had heard of before.

Unfortunately, finding all the files one needs is a real bear.

I eventually found what I needed, along with some instructions, here:
http://v1.archangelcastle.com/h4/ligue/patchanglais.php

Also, for the convenience of GOG users, I put everything into a neat package with some shortened instructions:
EquilibrisMod3.5.1.7z

I played through the Tutorial map with Equilibris installed, so everything appears nominal so far. Note that I could not find the modified campaign editor.

Also, for the convenience of GOG users, I put everything into a neat package with some shortened instructions:
http://www.filedropper.com/equilibrismod351

Thanks for files and especially for the instructions. Initially I just ran the .exe file which, of course, doesn't work. Without following your instructions, I doubt I would have figured it out.

Sadly, the download link seems to have expired. Any chance you could reupload it?

Thanks for the heads up. I have moved the file to a location that I hope will be permanently available. See my original post above for the updated link.

Thx a lot , gammaleak!
I'm glad to see that there's still love for Heroes IV :D

I never had any issues with the installation package I uploaded. I don't have GOG version though, so there might be some differences between versions. I'm basing this assumption on the provided heroes4.exe in the package. I'll upload the package gammaleak uploaded to my server, so anyone who has issues getting file I originally uploaded can get it working.
I'll edit 1st post with additional links.

Thanks, champs. Can't wait to try this.

Your link doesn't seem to work. I clicked on it, but when I click on the "download" button that is on the page nothing happens.