KDE 4 uses plasma to handle the desktop. Dolphin does not handle the desktop. If you are looking for a replacement for Nautilus, I would suggest using Nemo from Linux Mint. It is a fork of Nautilus that gets away from the Gnome trend of less is better by adding back what was available in older versions of Nautilus. Search for Nemo or Cinnamon at launchpad.net to be sure you are using the active or best PPA. To get the one available at this time, add the repository with this command in a terminal.
sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-stable
Unfortunately, installing it also adds cinnamon desktop. There is another fork of Nautilus, but I am not sure that it is available as a package. LXDE has a default desktop manager. If Nautilus has taken over your desktop, uninstalling it should return the default desktop file manager, PCMan. This is not the same as the window manager which should be OpenBox by default. Do not remove Nautilus if you use Gnome desktop or if you want a traditional desktop in Unity.