UPDATE: Canon has added support for most of their cameras to Digital Photo Professional 4. Now you don't need to install both versions to manage old and modern raw files.
EOS Utility (Canon's remote control software for cameras) has also 2 different versions: 2.x and 3.x.
DPP 3 is compatible with 32 and 64 bit operating systems and is linked to EOS Utility 2. This software is compatible with most Canon cameras and raw files, legacy and the most modern if you keep it up to date.
DPP 4 is compatible with 64 bit operating systems only and is linked to EOS Utility 3. This software is compatible with modern canon cameras and raw files. It is faster and has a better graphic interface than the previous version. Also have some extra features for editing and managing your images.
I believe Canon will be adding compatibility with older cameras and raw files to the newest software. As lots of photographers have images created years ago with old digital cameras. Although you can have both versions installed in your system (if your system supports 64 bit software), you will need to open a different application (with differences in the interface) depending on what camera was used to create the raw file.
At the moment the Canon 50d is only compatible with EOS Utility 2 and DPP 3. DPP 4 can't open Canon's 50d raw files.
DPP 4 can open Canon's 1100d raw files but the camera won't work with EOS Utility 3, you will need to launch EOS Utility 2.
The added compatible cameras by Canon appears to be a bit arbitrary, it is not related to the DIGIC processor used (50d and 1100d use the same), so it may be based on the age of the camera model.
DPP 4 is a very good improvement over DPP 3 and I'll be waiting for Canon to add support for legacy cameras. Meanwhile it will be great if Canon will publish a list of compatible cameras for each software (and keeps the list up to date).
