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In almost every case I can think of, the larger vehicles with four wheel power are 4x4, and the smaller ones AWD. As far as my experience goes, AWD is much more convenient, and also works at least as well on ice as 4x4 (at least 4h), and allows a rotational difference between front and rear axles, which 4x4 doesn't. I'm mainly talking about permanent AWD, but I suppose this question can apply also to driver controlled AWD.
So what's the reasoning behind this?
I really think that it's more of a naming convention issue but there is a marked difference between 4x4 and AWD.
In 4x4 all the wheels are "guaranteed" power. Power is sent from the engine to the transfer case and then the transfer case sends it to the front axle and rear axle. The reason I put guaranteed in quotes is because there is such a thing as full time 4x4 where the transfer case has a differential inside that allows slip between the front and rear. If you were to tie a 4x4 to a stump and pull until the wheels broke loose, then just before the wheel start spinning the front axle takes 50% of the load and the rear takes 50% of the load. When the wheels start spinning, at least one wheel in the front and one wheel in the back will start spinning at the same time. This is due to open differentials.
In AWD only one axle is guaranteed power while the wheels are not slipping. These systems tend to have a clutch of some kind that separates the axles. The clutch could be viscus, Subaru tends to use these. When the speed of the front axle does not match the rear the clutch will spin internally and heat up. When it's hot enough it locks the front and rear axles together. When it's cool enough it unlocks again. The clutch could be electromechanical, Mazda uses these. If AWD is allowed then an electromagnetic solenoid turn on half the clutch. When the speed of the axles does not match then second half of the clutch comes on to lock the axles together. The clutch could be built into a differential, Audi uses these. The power is split 60:40 and when a set of wheels starts to slip the clutches redirect power to the other axle. If you were to tie an AWD to a stump and pull until the wheels broke loose, then just before the wheels broke loose one axle will carry 100% of the load. When the wheels start spinning the axle with 100% of the power will spin for a short amount of time until the front and rear axles are tied together. When the axles are tied together the wheels that were spinning will slow down or stop and additional power will be required to break loose all the wheels.
Trucks need the ability to send power to both axles at the same time. This need comes from the fact that the true purpose of trucks is to do work. To quote a comedian "if you need to haul an airplane up the side of a mountain" you would use a truck. The guaranteed 50:50 split distributes the power evenly at all times and does not need to come on and off. The 50:50 split also makes sure that the strain is distributed, that neither the front nor the rear axle bare more strain than the other. There is a huge downside to 4x4. True 4x4 can not be driven on a dry hard surface for long. When driving there is a small difference in speed between the front and rear axle, especially in turns. This difference in speed builds up and causes the axles to fight each other causing tire scrubbing and jerking the steering wheel. As mentioned above some 4x4 vehicles mimic AWD by also having a differential in the transfer case. This differential can be locked and unlocked to go from part time 4x4 and full time 4x4. A major upside to 4x4 is that it can tolerate a 100% duty cycle. This means that the vehicle can be driven with constantly slipping wheels indefinitely. The transfer case builds no additional heat or bares any additional wear when compared to normal operation.
Some AWD systems can imitate true 4x4 action by positively locking the link between the front and rear axles. If that link is not positively locked then some modicum of slip has to occur before the system reacts. Marketing literature usually says that they react instant. There is no such thing as instantly, only really fast. "Really fast" means that for some time, no matter how small, one axle is favored over the other. Also AWD systems can not tolerate 100% duty cycle. Because the systems almost always employ some type of clutch mechanism, a build up of heat and wear will result from continuous use.
Trucks need the ability to send power to both axles at the same time. Why? Maybe I missed it. But once you've explained that this is the difference between 4x4 & AWD, the reasoning behind why trucks need to send power to both axles at the same time seems like the answer to the question.
One of the things the other answers neglected to mention and is probably the key point between AWD and 4x4 is that AWD transfer cases will almost always be a single speed differential. The 4x4 will almost always have 4-wheel high and 4-wheel low ranges and be selectable. This provides more torque if the driver deems the situation to need it. 4x4 may also have an "automatic 4wd" feature which ties in the hubs to the differential, but won't apply power to the front unless there is slippage at the rear wheels.
With this in mind, it is easy to see why there is the difference between the two and how they are applied by the manufacturer. It comes down to usage. Trucks and larger vehicles are used more towards utility: towing vehicles/trailers; pulling stumps. They sometimes require the extra torque needed to perform these tasks. Smaller vehicles with AWD are almost always used for passenger comfort and safety and don't require the extra torque. They have the AWD mainly for added traction.
NOTE: Like most things, please note the above is not an absolute, but general reasoning. Some large vehicles have AWD (ie: Hummer H2). I'm sure there are smaller vehicles with selectable 4WD as well.
4WD has the best traction in off road conditions, which is mainly what most larger vehicles are used for. As well as this, 4WD offers better fuel economy which is generally more favorable in a larger vehicle as these will already have a greater fuel consumption due to their added weight.
You are correct about AWD being at least as good in conditions such as ice. It is actually the best performer of the 2 in all road conditions due to its increased grip and traction. To answer your question about the smaller cars generally having AWD systems. You will find that these are usually sporty versions and an AWD system provides better handling. AWD also has a higher fuel consumption which is not really worried about in a sports car as its main selling point is to be fast, not have the best MPG.
You can read more about the differences here: http://www.digitaltrends.com/cars/whats-the-difference-between-four-wheel-drive-and-all-wheel-drive/
