The four door is marginally heavier, like 30kg or thereabouts.
This is a way small penalty to pay for the extra conveniance of increased access (yeah, yeah, dosen't look as cool, rah, rah - I have people in the back all the time).
The boot is also a touch bigger I believe.
Oh, and I love the fact most bogans can spot a Skyline, but the four door usually throws them