The Debian packaging for Samba 4 is maintained in the SVN 
repository of the Debian Samba packaging team. 

(parts of this file are from their README.building)

To build:

: Check out the official Debian packaging:

  svn co svn://svn.debian.org/pkg-samba/branches/samba4 samba4/debian

: Samba is not a native Debian package, so you will need to create
: an .orig.tar.gz tarball. Do something along the lines of:

  VER=$( dpkg-parsechangelog -lsamba4/debian/changelog | sed -n 's/^Version: \(.*:\|\)//p' | sed 's/-[0-9.]\+$//' )
  svn export samba4 samba-$VER
  ( cd samba-$VER/source && ./autogen.sh )
  ( cd samba-$VER/source && ./script/mkversion.sh VERSION include/version.h ../../samba4/source/ )
  tar zcf samba_$VER.orig.tar.gz samba-$VER

: With the .orig.tar.gz in place, you can now also export the debian/
: directory:

  svn export samba4/debian samba-$VER/debian

: Finally, build the package using whatever method you prefer:

  cd samba-$VER
  debuild

