So you've merged KDE 4 versions of your favorite packages and want to remove the 3.5 versions...
Make sure you have installed portage-utils and run the following:
emerge -avC `qlist -C -I -D -v kde | grep 3.5 | sed 's/^/=/g'`
qlist finds the duplicates and lists them, then we use grep to keep only the 3.5 versions, which we then pass to emerge (with = in front of each one) to unmerge :)
you can also remove all of kde 3.5 with a similar command:
emerge -avC `qlist -C -I -v kde | grep 3.5 | sed 's/^/=/g'`