Welcome the world of Jeeps.
Okay, the number of the lift 2, 4 or 6 (whatever) is the height of which the new suspension should put your vehicle over the stock height.
So, a 2 inch BB (budget boost or coil spring spacer) would put you two inches higher than stock.
The other heights that you'll see 3, 3.5, 4.25 or 4.5 and higher will give you at least that height over stock. Some kits start out higher and then settle down to the advertised height. Some start out at advertised and then settle to a little lower.
If you start adding a heavier front bumper, a winch, a heavier back bumper then, you'll notice spring settling or sag sooner.
Check out www.jeepsunlimited.com/forums
and read through the posts and archives. Check the member's rides thread to see what you like and don't.
Check out www.nagca.com
and read through that also. Check out the grand photos also to see what mods you like and don't.