The big difference between transparency Kwin3 and Kwin4:
Kwin3 transparency is rendered by software (KDE3), so gives a fake transparency on the Kwindeco (Crystal), menus and panel.
Kwin4 using the hardware (Nvidia/ATI,Intel chipsets) for real transparency (seems rendering by software is still possible).
For real transparency on KDE3 is Compiz or Compiz-Fusion needed.
JohnW