Part-time 4WD, not all-wheel drive
Most kei trucks use selectable part-time four-wheel drive. You drive in two-wheel drive on the road and engage four-wheel drive when you need it, usually with a lever or a dash switch. It is not a permanent all-wheel-drive system and should not be left engaged on dry pavement.
Low range
Many 4WD trucks add a low-range transfer case, which multiplies torque for steep ground, heavy loads and slow precise work. On a 660cc engine this matters more than it would on a large truck.
Rear differential lock
A factory rear diff lock forces both rear wheels to turn together. It is the single most useful option for mud, snow and loose ground, and it is worth specifying deliberately rather than hoping the truck has one.
When 2WD is the right choice
If the truck will work on sealed surfaces, a yard, a campus or flat ground, 2WD is cheaper to buy, has fewer components to maintain, and is often available in better condition for the same money.
Checking 4WD actually works
Engagement mechanisms seize on trucks that have sat unused. Ask specifically for confirmation that four-wheel drive and, where fitted, low range and the diff lock all engage and disengage.
This is general information, not legal or customs advice. Import eligibility and road registration are decided by the authorities in your country, state or province, and rules change. Confirm your own position with your customs broker and vehicle registry before committing to a purchase.