Bonjour Benoit!
ConferenceXP can easily handle a conference between 4 different sites. However, there are some qualifications...
1. You need sufficient network bandwidth - approximately 2mbps upload per node and N * 2mbps download.
2. You need sufficient hardware for a node - we run dual proc or dual core machines, with hyperthreading, a nice ATI or NVidia graphics card, 1GB RAM, etc.
3. You will need cameras and audio devices.
4. Ideally, your sites are multicast connected. Otherwise, you will need to install and use a Reflector.
We allow you to configure the encoding bit rate, which becomes a trade-off between CPU utilization, to get the encoding done, and network bandwidth. If you have high end machines and sufficient bandwidth, you run 640x480 compressed video at 30fps and send uncompressed audio.
If you need more information, please let me know; be sure and provide some more information about what hardware you have, etc. so I can provide better answers.
JVE