So, enable wheezy-backports, e.g. put this in your /etc/apt/sources.list
deb http://ftp.iinet.net.au/debian/debian/ wheezy-backports main contrib non-free
Then install libreoffice 4:
sudo apt-get install libreoffice -t=wheezy-backports
That's it.
Post-script:
To see what your policies are:
apt-cache policy libreofficeand as it turns out the backports repo only has a score of 100, and looking at man apt_preferences:libreoffice: Installed: 1:4.0.3-2~bpo70+1 Candidate: 1:4.0.3-2~bpo70+1 Version table: *** 1:4.0.3-2~bpo70+1 0 100 http://ftp.iinet.net.au/debian/debian/ wheezy-backports/main amd64 Packages 100 /var/lib/dpkg/status 1:3.5.4+dfsg2-0+deb7u2 0 500 http://ftp.iinet.net.au/debian/debian/ wheezy/main amd64 Packages
If the target release has been specified then APT uses the following algorithm to set the priorities of the versions of a package. Assign: priority 1 to the versions coming from archives which in their Release files are marked as "NotAutomatic: yes" but not as "ButAutomaticUpgrades: yes" like the Debian experimental archive. priority 100 to the version that is already installed (if any) and to the versions coming from archives which in their Release files are marked as "NotAutomatic: yes" and "ButAutomaticUpgrades: yes" like the Debian backports archive since squeeze-backports. priority 500 to the versions that are not installed and do not belong to the target release. priority 990 to the versions that are not installed and belong to the target release.
To see what's available in backports, do
aptitude search ~Awheezy-backports