I'm looking for a way to get remote access to KDE4, including KDM because I need to login first, do you know any way how this could be done?

Unfortunately, as KDE is not yet running, it cannot really help you.
However, it is possible to setup X to permit remote access (via protocols such as VNC). Depending on how this is setup, it can either launch a new session, or manipulate the primary console.

You can do that using the xdmcp support in Xorg, kdm has support to xdmcp too, but not from the GUI, you need to edit configuration files manually. But that will be very slow if you do not use a fast network (100 Mbps). Do not even think in using it with wifi, xdmcp hates lost packets in the connection. You can try nomachine, which is a modified (comercial) xdmcp protocol that copes with the packet loss and amount of round trips in xdmcp. The fast network I mentioned earlier is needed because of amount of round trips xdmcp uses.

You may also wish to try VNC. I recall using a VNC server which was started as part of the X startup process and allowed me to remotely control the system.