Yeah, KDE 4.0 wasn't bad because of its design philosophies or functionality. Everytime people hear that phrase, they instantly jump to that conclusion for some reason. No, it was resource hungry and crashed a lot (mostly due to aforementioned resource needs).
The later versions of KDE4 didn't change much, they just pared down resource requirements, stabilized things and let the average computer specs catch up to them. Now KDE is considered the more "conservative" DE.
KDE4 was latter half of 00's. Oh, the bitter memories of having your CPU and memory getting hogged by the mysterious daemon called Akonadi. Or was it Nepomuk? At that time, many waved away concerns that Linux was simply using the unused memory for caching stuff, which is why the resource usage viewer displayed very small free memory amount, so they say. Well, that didn't explain the excessive swap thrashing that was occurring in the background.
Hunted for workarounds only to find out that disabling the above services also disables some essential desktop features, and made me rethink whether the switch from GNOME 3 was worth it. Sure, GNOME 3 had horrible UX, but at least my PC still had resources to do other stuffs then. But I sure missed GNOME 2 at that time.
The later versions of KDE4 didn't change much, they just pared down resource requirements, stabilized things and let the average computer specs catch up to them. Now KDE is considered the more "conservative" DE.