The FreeBSD packages for KDE4.1 are now available on the FreeBSD FTP servers. If you don’t want to build from source you can now use these packages to quickly install a KDE 4.1.0 desktop.
to install (as su) KDE4.1 enter:
pkg_add -r kde4-4.1.0
If you’re not familiar with installation of FreeBSD packages, or if you want to brush up your knowledge, have a look at Using the Packages System chapter from the FreeBSD handbook.