In other words, this site is pretty safe...
Apparently from the different conversations I had it is possible to install CS4 and CS5 on the same mac.
How ever it is not possible to install different flavors of photoshop on the same machine. (like Web and Full versions)
I personally don't see why you would want to have different version of Photoshop running on the same machine, and it is also not recommended at all.
Photoshop is not just a normal that is installed and have all its files in one place. It installs Frameworks and tones of preferences. CS4 and CS5 seems to be compatible with each other.
If anyone tried it, please let us know how it goes.