Guild Wars 2

My armor has Major Rune of Infiltration (1/4)

1. 1 is filled in; blue text. Number 2-4 are greyed out. Can you slot other runes to make the number 4/4<

Yup. That’s how Runes work. You nailed it.

Also this isn’t really a crafting question.

To clarify: you need to slot other instances of this rune on other pieces of armor.