H2-H4 is the normal pavement driving option. You press the button on the shifter to toggle between 2wd and 4wd. When you are in 2wd all the power goes to the rear axle only. When you are in 4wd the transfer case is not slipt 50/50 front to rear, it can allow 100% power to go to any of the four wheels. This is also known as All Wheel Drive. The transfer case can do this because it has a differential built in (center diff).
H4L means high range, 4wd, locked center diff. The transfer case is now locked to a 50/50 power split and you are guaranteed to have power going to at least one front and one rear wheel at all times.
L4L is low range, 4wd, locked center diff.
The center diff in your 4Runner is not an extra traction aid, it doesn't give you any off-road advantage, it just gives you an option for on-road improvement.
When people refer to lockers,they are talking about front and/or rear differentials, not the center one.