In Delphi 4, when specifying search paths, you can use $(delphi) to refer to Delphi's installation directory.
If your are writing an installation program, you may be interested in how to find out, where / which version of Delphi is installed.
It is in the registry at:
HKEY_LOCAL_MACHINE\SOFTWARE\Borland\Delphi\3.0
or
HKEY_LOCAL_MACHINE\SOFTWARE\Borland\Delphi\4.0