Hi,

For linux install both QT3 / QT4 in separate directories. You need to setup QTDIR, PATH and LD_LIBRARY_PATH. Best is to create a small shell script.
Assuming that QTDIR is /lib/qt4

First set the QTDIR
export QTDIR=/lib/qt4/

Then set the PATH
export PATH=$QTDIR/bin:$PATH

then set the LD_LIBRARY_PATH
export LD_LIBRARY_PATH=$QTDIR/lib


if you switch to qt3, do the same but then QTDIR set to the installation directory of qt3