I would think that you couldn't get much lighter that the HB-4 kit.
I was going to suggest a headless option as well. In fact, PBG does have a Steinberger bass kit ... the SHB-4 (guessing you are wanting the Spirit-type model).
As Simon suggested, a wide strap goes a long way in helping with heavy instruments. My custom 6-string bass weighs in at about 10 lbs, or 4.5 kg. My 3" wide woven fabric strap makes a three-hour gig a breeze.