What are 4x4 lockers?

There are 2 types of Lockers...

Since you specially said 4 x 4, I think your referring to locking hubs. They are devices on the front axle hub of a 4 x 4 vehicle that engages both front tires to provide maximum traction when in 4 wheel drive. Without the hubs locked the vehicle's front wheels are not engaged/being driven, even with the transfer case in 4wd. The vehicle will also drive/turn easier, especially on the street when the lockers are "unlocked". There are manual/electric lockers avail., manual being the choose of serious off-roaders, they are more durable but you have to get out of the vehicle an manually engage them.

There is a different "lockers" available/used in axle housings. It locks the both wheels together on that axle to providing maximum traction. This locker can be used on 2wd vehicle, specially in drag racing. A locker by definition is either/can be 100% on or off, so I don't consider a Limited slip, Posi units a locker, neither is a spool which is 100% always locked. There a 2 true lockers, air operated like the one from ARB or the original Detroit locker. The air operated unit is controlled by the driver, while the detroit locker is automatic. The Detroit unit unlocks during turns, when not under power, but can lock up unexpectedly when under power exiting a turn .