I did find him and killed him but I am now wondering the same. What happens now?

Is there any indication I killed him? And if I now find and kill the other 3 I get this achievement?
There is no mission attached to this achievement I believe. When there is a mission (like these daily ones where you have to kill x amount of spiders) then you you see you have done x out of y kills.

Nothing like this with these 4 Waldo's?