The annoyed ranger was probably just tired of pulling stuck tourists out.
The road gains 800 vertical feet (243.84 m) in 0.6 miles (0.9 km) at a 25% average grade, with steeper grades in sections. This is a paved public road but it is open only to 4 wheel drive vehicles. It is the steepest road of its length in the United States.
There's quite a few roads in the UK with 1:4 gradient, and one just down the road from me at 1:3 whose only restriction is no trucks and vans. Going downhill on the motorbike makes it feel like you're falling off! :)
There's an annual charity event that involves the rolling of over 30,000 Jaffas (spherical confectionery-coated chocolate confectionery) down the hill.
The video posted in another comment doesn't look that bad.
I am not sure about the grade, but one of the Bangalore's weekend gateway has some steep sections(a low average grade but some sections are steep).
https://www.youtube.com/watch?v=C0_MyaPzM88
That footage is from a rally but every weekend, fwd cars with 800/1000cc engines and a peak torque of about 70 Nm do it just fine. Most of the cars have 2-5 occupants decreasing the already low torque/kg, and almost the whole climb is done in a bumper to bumper traffic(no momentum for the climb).
https://dc-cdn.s3-ap-southeast-1.amazonaws.com/dc-Cover-nj6b...
However, in the general case throughout most of the US West, "4WD roads" do indeed tend to be as much about having high clearance as about having 4WD per se. It's also just a way of saying that, if you just have a random rental car and aren't used to driving very rough and often narrow/exposed roads, this is probably not a good idea.
The Subaru Outback (AWD) has 8.7" of ground clearance. The Ford Explorer ("Intelligent" 4WD) has a ground clearance of 7.8".
If "Intelligent 4WD" means some kind of differential, then it really is just a marketing term at this point, though I learned about the distinction between 4WD and AWD when I bought a Subaru.
4WD is meant for off road conditions. You get more torque so you'll be able to get over rocks and out of ditches. The downside is you shouldn't use it over 10mph or so.
I can't think of any recent car that is 4WD. It doesn't make sense since you'll probably never be "off road". Cars don't have proper ground clearance.
That's not strictly true, and varies between implementations/transfer case designs. In Jeep Wranglers, for example, which have both high and low gearing options in 4WD, the owner's manual will indicate that you can drive at any speed (on loose surfaces such as sand or dirt) in 4HI and 0–25 MPH in 4LO.
A friend of mine had a old truck that would randomly switch to 4LO on the highway. It's pretty scary driving 60 and having 4LO switch on.
The inside wheel would spin if the outside doesn't slip.
There are many types of SUVs and trucks with 4wd in the US. I guess in Europe you only have Land Rovers or G-wagons. I don't think GLs or MLs are common in EU but you can get beefed up off road packages in the US.
Source? Center locking diffs are more or less standard, rear lockers are options in LR3/4/post 2004 Disco's, Sports and Range Rovers (although standard in some, I think the Supercharged and possibly the SVRs, but I'd need to check) Front lockers are not factory options and I don't ever recall seeing after market lockers for these models advertised.
Defenders, Discos upto 2004, Range Rover classics (and probably P38s) and all series vehicles have the standard after market options for front and rear lockers.
http://www.ebay.co.uk/itm/like/182246825443?lpid=122&chn=ps&...
Center diffs are for the most part (in Land Rovers) locking differentials, though some Discovery IIs had open center diffs and things like Freelanders use a viscous coupling in place of a mechanical diff lock.
Newer models have an electronically controlled locking diff in the gearbox (center) and an optional electronically controlled locking differential at the rear.
But front diffs on modern Land Rovers are open.
Edit: Here is an image showing the locking diffs on an LR3/Discovery 3: http://www.landroversonly.com/forums/attachments/f41/37257d1...
Note the center diff is unlocked (or open) and the rear is locked, there is no option to lock the front, which will always be open.
I was puzzled by this as I was stopped by a ranger coming out of Waipio Valley on Hawai'i, where there was a sign "4WD only" and my rental car had AWD. I thought they were the same, just different names, so I argued with the ranger that I do in fact have 4WD, which is in my case just called AWD and in other cases 4x4 as well. Well, he was still pretty upset but the "I am a European and we have different cars" finally disarmed him...