HOMM4 was the last HOMM game done by New World Computing & 3DO. After that, 3DO went bankrupt, Ubisoft ended up with the Might & Magic name, and they did a reboot to put all M&M games in their new (generic and boring) world of Ashan.

HOMM4 (and its expansions) is the last strategy game in the old NWC Might & Magic universe. Might & Magic 9 is the last RPG game in that universe. HOMM4 probably/maybe comes first in that universe's timeline, then M&M9, then the HOMM4 expansions. But at that point, I'm not sure NWC was doing as much to integrate the stories as they had in the HOMM2-3 / M&M6-8 era.

This is the chronological order of the games in the original universe.

Heroes Chronicles: Warlords of the Wasteland
Heroes Chronicles: Conquest of the Underworld
Heroes Chronicles: Revolt of the Beastmasters
Heroes Chronicles: Masters of the Elements
Heroes Chronicles: The World Tree
Heroes Chronicles: The Fiery Moon
Heroes of Might and Magic: A Strategic Quest
Heroes of Might and Magic II: The Succession Wars
*Heroes of Might and Magic III: The Shadow of Death
*Heroes of Might and Magic III: Armageddon's Blade
*Heroes of Might and Magic III: The Shadow of Death
Heroes Chronicles: Clash of the Dragons
*Heroes of Might and Magic III: The Shadow of Death
*Heroes of Might and Magic III: Armageddon's Blade
*Heroes of Might and Magic III: The Restoration of Erathia
Might and Magic VI: The Mandate of Heaven
*Heroes of Might and Magic III: The Restoration of Erathia
Might and Magic VII: For Blood and Honor
*Heroes of Might and Magic III: Armageddon's Blade
Might and Magic VIII: Day of the Destroyer
Heroes Chronicles: The Sword of Frost
------ Post Reckoning
~Might and Magic IX: Writ of Fate
~Heroes of Might and Magic IV
#Crusaders of Might and Magic
#Warriors of Might and Magic
#Shifters

~MM9 and HoMM4 take place around the same time but on different continents.
*The HoMM III campaigns run at quite a few different times.
#These games are spinoffs and take place on Ardon but are still in the same universe and take place around the same time in the years following MM9 and HoMM4.

I put this together a few months ago from information at the Might and Magic wiki.

Does anyone know where Might and Magic X falls in this chronology? I heard it goes back to the story-lines from M&M1-5.

Might and Magic X is not set in the old MM universe made by NWC. It is set in the new world created by ubisoft when they got the rights to the series. The only thing about MMX that is related to MM1-5 is the gameplay which is inspired by the old style of gameplay.

While MMX certainly does take place in Ubisoft's Ashan world, the MMX teaser trailer when they first announced it made reference to Alamar. It remains to be seen whether or not they will attempt to blend in some of the old fiction or if that was just a way to get fans excited (probably the latter).

Many characters, places, or even events in the new universe use names from the original NWC universe. So while there are familiar names for example in HoM&M V, those characters has nothing to do with the original characters of the same name.

I will never find it again, but the developers stated at some point that nothing in Ashan will ever point back to any part of the New World Computing storylines. And that all sci-fi elements are removed from Ubi's M&M universe.

All that said, the fans are craving it, so they might pull a Microsoft.

Yup. At the end of the day, they want to make $$$ so who knows what they'll do. :^)