I don't have access to my pre-4th edition GURPS books right now. But I was thinking about the +4 to target a hex with an area-effect weapon such as a grenade or an explosive spell.
I think it originated in either GURPS Fantasy (for 1st edition GURPS) or GURPS Magic (for 2nd/3rd edition GURPS, the one with the dueling magicians on the cover.) But I can't check from where I am.
Can anyone reading this take a look and see where it originated?