Who were the four that stood at door in song Children Go Where I Send Thee?

In the song "Children Go Where I Send Thee," it is mentioned that two stood at the door, not four. Here is the excerpt from the song:

In the Lord's army there's a little band,

Two are marching hand in hand,

Christ will always lead them on,

These two he calls His own.

So children go where I send thee,

I'm the one who's sending thee,

Go tell them that I love them,

And I will always be there.