I installed qt4 from qt-x11-opensource-src-4.1.2.tar.gz and its libraries gets installed in /usr/local/Trolltech/Qt-4.1.2/lib/. Now how can i give this new instalattion on my RHEL 4 system that have also QT-3.3-1 installed. I want to run latest kopete 0.12-beta2 application but its failing on
checking for Qt... configure: error: Qt (>= Qt 3.3 and < 4.0) (headers and libraries) not found. Please check your installation!

i also added that path in /etc/ld.so.conf and run ldconfig to result /etc/ld.so.conf as
include ld.so.conf.d/*.conf
/usr/local/Trolltech/Qt-4.1.2/lib/
/usr/local/lib

But still same error how can i make it to use that installation?

You need to tell the configure script where to look. Use an argument like "--with-qt=/usr/local/Trolltech/Qt-4.1.2". Not sure exactly what it should be - try "./configure --help" and it will probably show you how to say it.