Is there a difference between all wheel drive and 4 wheel drive ?
Short version: AWD is usually an always-on system that can be used on dry pavement whereas four wheel drive is a system that is activated either automatically or manually and can only be used in slippery conditions. AWD vehicles have some sort of device that can control how many rotations are going to the front and back axles, wheras with 4wd it’s a fixed ratio.
However, the two terms have been used more or less interchangably at various points in time. A lot of modern AWD or full time 4wd systems also don’t neatly fit into either category, either. In general, though, AWD tends to be on cars and car-based SUV’s whereas 4wd is on truck-based vehicles. There’s also a huge amount of variation in how each system works, especially with AWD, so you really have to get into the nitty-gritty of how things work on each vehicle.
Most 4 wheel drive vehicals have control of when all 4 wheels are used.
All wheel drive is always 4 wheel and nothing has to be done by the operator to have all wheels driving.
Most modern 4 wheel drives are now electric shifted into 4 wheel unlike the older vehicals like a 4 X 4 pickup that would have a mechanical shift lever in the cab to engauge the transfer case to drive the front axle. Some also had manual lockouts on the front wheel hubs to disengauge the wheel to differental connection.
The front differental in my 4x4 pickup is BO now and now I have a rear wheel drive pick. 
My understanding of the terms has always been that 4X4 almost always includes a lower selection of gearing, that is 4X4hi (which has its set of gears) and 4X4lo (which has its own.)
My Samurai has an extremely low set of gearing when it is in 4lo. I can practically act as my own spotter.
Four wheel drive typically indicates some system that has a manually actuated transfer case which engages the non-drive axle (typically the front, but there are some front wheel drive cars with 4wd transfer) and has full locking differentials. Since an axle with a locking diff will not allow the wheels to rotate at different rates it cannot be should at highway speeds or on dry pavement. Many also have a front/rear locking diff and should not be used in dry conditions at all. Many 4wd vehicles also have a seperate low range shaft that allows for high torque output for offroad and heavy towing at low speeds.
All wheel drive generally indicates a system in which all four wheels are engaged (or are capable of being engaged) at all times. Such systems have limited slip or open differential permitting it to be driven at highway speeds and typically use some kind of actively controlled clutch system to distribute power.
Stranger
Edmonds has a a good article explaining the different wheeling driving arrangements.
It’s important to note that the technology is very distinct between the two. IOW the terms 4x4 and 4WD generally refer to ‘off-road’ style vehicles (usually pickup trucks & SUVs) that have the following features:
[ul]
[li]A separate transfer case bolted onto the end of the transmission (the tranny can be automatic or manual, it doesn’t matter).[/li][li]Four separate gears or ‘modes’ for the transfer case: 2W high, 4W high, Neutral, and 4W low (really old 4x4 trucks also used to include 2W low)[/li][li]Transfer cases used to always be switched physically via a shifting lever next to the transmission shifter, nowadays most all are switched with electric servos controlled by a switch on the dash.[/li][li]Sometimes the rear wheels have a limited slip differential, but it’s almost always another option on top of 4WD that must be ordered separately.[/li][li]Front axles can almost never be ordered with a limited slip differential anymore (again, older trucks could).[/li][li]4x4 vehicles almost always have an off-road ‘look’ to them i.e. aggressive tire treads, higher ground clearance, skid-plate protected under carriage etc…[/li][li]4WD vehicles are still predominantly conventional drive trains: Engine **NOT **mounted transversely, transmissions NOT transaxles, 2WD mode is usually always rear wheel drive.[/li][/ul]AWD vehicles on the other hand usually have none of the above except for possibly a rear limited slip differential (which, again, must be ordered separately). They instead feature:
[ul]
[li]The all-wheel-drive aspect is built right into their transmissions/transaxles.[/li][li]The AWD ‘mode’ is constantly on or controlled by a computer (rarely by the driver).[/li][li]They can be any style of vehicle: Compact car, 4-door touring/sport sedan, minivan, crossover (a small SUV).[/li][li]They can have conventional or (more often) transverse mounted engines/drive trains.[/li][li]The all wheel drive is not merely for traction in snow & rain, but rather for all around better performance & handling even on dry pavement.[/li][/ul]Note that many formerly 4WD vehicles have now become AWD. For instance the Nissan Pathfinder was a standard 4x4 SUV with a driver controlled transfer case with a high & low range. It’s now become an AWD family truckster rather than an off-road SUV (the Nissan Xterra now fills that role).
The Society of Automotive Engineers defines an all-wheel-drive vehicle as one with an ‘on-demand’ feature that occasionally or intermittently sends power to the non-primary powered wheels. The majority of these systems are based on a front-wheel drive platform that has a differential capable of sending some power to the rear wheels when needed. This transfer of power is handled electronically, usually when one or both front wheels begin to lose traction.
All-wheel-drive can also be used for more technically advanced, high-performance purposes. Acura has a system called Super Handling All-Wheel-Drive or SH-AWD available on some models. The system detects wheel speed and slip and transfers power front-to-rear or side-to-side to maximize cornering grip and provide class-leading handling.
4WD or 4x4 systems have a dedicated transfer case including a separate low-range to provide a slower, more controlled delivery of power. Low range essentially acts as a torque multiplier to provide exponentially more ‘grunt’ to climb over obstacles, up steep grades and handle real off-road driving.
So 4WD or 4x4 systems are capable of handling much rougher terrain and off-roading. AWD systems provide additional on-road traction when needed and usually operate as front-wheel-drive systems under normal driving conditions.
Full time 4x4 acts like an AWD system, but it also has Low Range, whereas AWD only has High Range.
My AWD SUV sends power to the wheel(s) that have the best traction and a button that will send the power to all 4 wheels marked (unsurprisingly) 4 wheel drive.
That’s not quite right, is it? Modern 4x4 systems can be used on dry pavement, but you probably just shouldn’t due to killing your gas mileage.
My previous SUV was a 2007 Ford Explorer Eddie Bauer and it was available with 4WD or AWD for the same price. I chose 4WD solely for the Low Range. I nicknamed it “The Mountain Goat” because of how well it performed in Low Range…literally climbing very steep, rocky, rutted inclines without a hiccup, just like a real mountain goat!!! 
I traded it for in for a Crossover last November, a 2012 Mazda CX-9 GT and decided to go with the front wheel drive model instead of AWD. The AWD system cut fuel economy by 2mpg city and highway and it cost $2k more to purchase. Due to the additional weight and increased rolling resistance of the AWD, it was almost a full second slower from 0-60mph.
So far, I’m very happy just having FWD. It has a great traction control system and stability control that kick in if needed on wet roads, but they are rarely needed. I’ll never take it on some of the terrain that my Explorer could handle, but as a tradeoff I can go from 0-60 in 7.1 seconds! :D:cool:
Plenty of them still have a direct connection between front and rear axles, so they will still bind if 4wd is used on dry pavement. (because the front axle always travels farther than the rear axel) This puts a lot of stress on the driveline.
These were popular in the 70s, my brother had a 70-something Chevy Blazer with full time four wheel drive. Basically the transfer case also acted as a differential between the rear & front drive shafts. If the rear wheels slipped the front drive train would engage automatically. They weren’t too popular as they got terrible gas mileage and required front lock-out hubs if you wanted to manually disengage the front axle.
My 76 Chevy pickup is like this. It will feed a little power to the front end even in ‘2’ wheel drive. Or you can lock it in for true 4x4 or lock in 4x4 low. It does not have manual front hubs BTW.They are always locked in.
Yup. My 2006 Pathfinder should not be in 4x4 when on dry pavement. Though if it’s straight line driving, and say about half of it is snowpacked I’ll do it. My Wifes’ 03 Grand Jeep has an option for ‘Full Time’ 4x4 that allows you to have it in 4x4 on dry pavement.
Also for consideration, the old Honda Civic Wagovans and the Subaru Loyales, which were both definitely what we’d call all-wheel drive cars, but had low ranges. The Subaru was really ambigious because I think they were badged as “4WD” but they were in this futuristic 80’s font where the 4 really, really looked like an A.
It’s really hard to construct a definition that works in all cases.