sh-3.2# pkg-get -i gcc4core

Ends with:

## Executing postinstall script.
*** NOTICE ***
Fixing the system headers
for a detailed log see /var/sadm/install/logs/CSWgcc4core-20090205023023
Don't forget: whenever your system headers change run the mkheaders script!
pkgadd: ERROR: postinstall script did not complete successfully

Installation of <CSWgcc4core> failed.
ERROR: could not add CSWgcc4core.

However the binary is installed and will work if called: /opt/csw/gcc4/bin/gcc

The problem appears to be this broken symlink:
http://osdir.com/ml/os.solaris.blastwave.user/2008-01/msg00043.html

bash-3.2# cd /opt/csw/gcc4/lib/gcc/i386-pc-solaris2.8/4.0.2/include/
bash-3.2# ls -ld X11 
lrwxrwxrwx   1 root     other         28 Feb  5 01:21 X11 -> root/usr/openwin/include/X11

Fix this broken symlink with:
bash-3.2# ln -s /usr/include/X11 X11


Add to path: /opt/csw/gcc4/bin/
bash-3.2# PATH=$PATH:/opt/csw/gcc4/bin/


Download Mercurial Source and install with:
http://www.selenic.com/mercurial/wiki/index.cgi/BinaryPackages#head-f90bf04a392bc29d5e90494a6e753bc8f140dbf3
bash-3.2# /opt/csw/bin/python2.5 setup.py install --force

bash-3.2# cd /opt/csw/gcc4/bin/
bash-3.2# ln -s /opt/csw/gcc4/bin/gcc ./cc