To count as a 4000'er for this list, you can drive as far as is legal, then you must proceed on foot (no bikes). For a gated road that means you drive to the gate, and hike or ski from there. The fact that the road may be ungated in the future or was ungated in the past isn't relevant, it must be drivable on the day of the hike. The trailheads start at the gate.
The reason this rule has gotten a bit hazy is that it's legal and possible to drive a jeep on some woods roads much further than a standard passanger car could go, allowing some intrepid drivers the ability to shorten their hikes. But this really doesn't apply to gated roads, where the rules seem pretty clear.
This may not be written on the FTFC site, but it has been discussed by the committee a few times on the AMC BBS. I may have faulty recall, it wouldn't be the first time. I agree, that writing this on the FTFC site could clear things up. The piece that is missing is that it must be both legal and passable for a car in order for you to use a bike. The gate would rule that out.
-dave-
EDIT: On second thought, I'm not so sure anymore if I recall the conversations from a few years ago correctly. A clarification would be useful.