Light Infantry in skirmish formation can be in a 2x2 formaton or they could be in a 4x1 formation. However if they are in a 4x1 and they are not defending a hill or obstacle they will reduce the number of their dice by 1 whenever they take a Cohesion Test.
In skirmish formation they are represented by 4 bases with 3-4 skimishing figures on them, They can also be in tactical where the front two bases are the skirmish type and the rear two are normal bases with 6-8 figures.
It is also worth remembering that while an extended skirmish line gets double dice for close range shooting, it still only gets the normal 5 shooting dice at medium range - and it is likely that these will need to be split between more targets.
I only use 4x1 extended skirmish lines if I need to screen 2 of my own units with them.
Thing to remember is that skirmish is not a tactical formation. A unit in skirmish is skirmishing period, regardless of how deep or wide the unit is. They stop being in skirmish once they assume a tactical formation again, usually represented by replacing half the skirmishing bases with normal unit bases (figures in two lines and more figs per base).
In one of our games the Frenchies deployed a unit in skirmish formation in a column, hoping to pull a coup de something until I pointed out that the unit was in skirmish formation and not in a column, even if the bases were lined up one behind the other. So much for getting a double move down that road, I outclevered those French types that time.