If you downloaded the linux package, it should work without extra dependencies.
That guide is incorrect. There's no such thing as libqtgui4-gui. The required library is just
libqt4gui, which you should already have anyway.
Thank to both of you. Indeed I seem to have libqt4gui, but if I try to run bitcoin-qt I get:
./bitcoin-qt: error while loading shared libraries: libQtNetwork.so.4: cannot open shared object file: No such file or directory
Installing libqt4-network finally worked for me