You add 2 to the number rolled on the die.
So technically, as worded, you wound on a 4+, but if you roll a 2 you then add 2 to make it a 4 (which succeeds).
This is techically somehow distinct from a weapon which always wounds on a 2+, and I'm sure theres a mechanic somewhere where this would actually be useful (or bad).
Whatever number is on the die, add 2 (except against vehicles).