4 Daggers can kill a Welder easily.
In this case there were a few factors:
One was injured
Two came in ahead of the other two
Two initially targetted a mex
The mex explosion blew up one and damaged another
One Dagger decided to go for a walk somewhere else mid-battle
A kodachi arrived and killed two of them

Daggers have by far the worst DPS of any raider and a Welder is roughly equivalent to a lotus in terms of the number of raiders needed to kill it.

Foie gras.