Okay let's get this thing straight. Subarus have three differentials.
The front differential (an open diff) controls power delivery between the front wheels only.
The center differential (LSD- vicscous coupling on manual tranny, electro-mechanical clutch on auto) controls power delivery between the front and rear wheels.
The rear differential (open in most models, LSD in some) controls power delivery between the rear wheels only.
if the front wheels are slipping (it only takes one since it's an open diff), power is transferred by the center diff to the rear wheels and vice versa.
In the case where the front and rear differentials are both open, the car will lose traction if just one front and one rear wheel are both slipping.
In the case where there is a rear LSD, the car will only lose traction if one front wheel and both rear wheels have lost traction.
In the case where there is a front and rear LSD, the car will only lose traction if all 4 wheels are slipping.
4WD is like AWD but with a locked center diff, meaning that the power sent to the front wheels is always equal to the power sent to the rear wheels. Of course, Subaru used to call their AWD systems "4WD" until the early 90s, and I think the Brits still call it 4WD.
edit: Tim, Jon, you beat me to it. Jon, I don't think they need to be diagonal wheels for the car to slip. With the open diffs, any 1 front and any 1 rear will do it.