I just downloaded and compiled KDE trunk in my $HOME folder, but now I need to know how I can get KDevelop4 to compile my program against KDE trunk.

How can I change the KDE path in KDevelop4 to $HOME/kde? And I have $HOME/qt, but I don't know where that fits in.

This is usually done by setting KDEDIR, PATH and LD_LIBRARY_PATH prior to starting KDevelop.

Would this only change where g++ would look for KDE in my current project?

This would change where g++ looks for everything, for all projects opened in the KDevelop instance which you ran with those variables set.