Mark Packages by Task in Synaptic.

Marking packages by task is a feature available in the Synaptic Package manager ,which makes installing complete desktop environments and other packages very simple in ubuntu.Suppose you want to install KDE or Xubuntu in you Gnome Desktop Environment.Then,instead of searching for individual packages and marking them separately,you can use the Mark Packages By Task option.It gives you some predefined options such as installing different desktop environments,servers and even 2d/3d editing suites.Simply mark a task and apply.Synaptic would itself mark the individual packages and install them.This is very handy feature that saves a lot of time.
Use this feature to install the KDE 4 package.