Here's why:
* Lightsworn monsters have a specific summoning condition: They require you to have a certain number of Lightsworn monsters in your graveyard to special summon them.
* The summoning condition varies: Different Lightsworn monsters require different numbers of Lightsworn monsters in the graveyard. For example, Charge of the Light Brigade needs 5, Lumina, Lightsworn Summoner needs 3, and Lyla, Lightsworn Sorceress needs 2.
* You need to meet the specific condition: You can't just summon any Lightsworn monster with any number in the graveyard. You must have the minimum required number for the specific monster you want to summon.
Therefore, even with 4 Lightsworn monsters in your graveyard, you can't automatically special summon 2 monsters. You would need to have at least 5 Lightsworn monsters in your graveyard to summon Charge of the Light Brigade, for instance.