Please tell me what are the prerequisites for installing xfce.
If you use "rpm --test -i xfce*.rpm" or yum without the "-y" flag and do "yum install xfce" (or "yum groupinstall xfce"?) you'll find out easily yourself. AFAIK you'll need GTK, TIFF, JPEG and PNG support libraries and most of the standard Xorg/X11 libraries. Else what's against visiting the XFCE site and reading the docs?
Also tell me is it possible to install xfce on FC4 without root permissions
Doesn't make sense since you need to add the DE settings to the X11/Xorg configs in /etc/X.* and need to update the RPM database and both are the domain of root. Maybe if you compile from source and install in ~/ or "rpm --relocate" there's a way around this using personal X resource files (think ~/.xinitrc, ~/.xserverrc etc, etc) but that's not something that can be considered "standard", failsafe or maintainable.