2H is 2 wheel drive high
4H is 4 wheel drive high range,
and I think u got it wrong, its probably 4L not 2L, that means 4 wheel drive low range.
When ur driving on normal city terrain, tarmac etc u would drive on 2H,
when ur going through off road terrain, sandy to soft, and moderately wet mud etc u need to be in 4H, this engages 4 wheel drive in a higher range, which means that all 4 wheels will be sent equal power at a moderate level, depending on you vehicle, torque transference and engine, 4H is usually enough for most off roading,
4L is a low range gear, again it will engage all 4 wheels but will give them alot more power, this is good for step inclines around 30 degrees and more in very soft terrain or wet sludge and mud.
It seems like you're relatively new to off-roading, what vehicle did u see these gears in??? For starters 4H is probably enough for you,
a word of advice, never go off-roading alone, ideally have atleats one more person with you, keep a spade with you,
its ideal if you can have another jeep to keep you company, tow you out of a spot etc.
Do let us know what vehicle you're talking about and post pics if you have some.