I agree with what ga_edwards stated. Very little neck dive on the 4, and any issue I have with the bass while seated is more about the small lower horn than weight issues. With the bass strapped on I have no dive - I don't have to support the neck at all while playing.
My 5 also has no neck dive issues, but at 8lbs. even, I have a feeling it's on the heavy side for one of these basses.
I don't have neck dive issues with my 5-string however when I play sitting down it still is on a strap. Mine weighs less than 8lbs.