The other day I asked about the origins of the +4 to hit a hex.
No one chimed in with the answer, but now I'm back at my office and can check myself. It's exactly where I thought it was from.
The earliest reference I could find was in GURPS Fantasy (subtitle - Magic System and Game World) for 1st edition GURPS. Or maybe for 2nd edition - I had 1st edition, and I got this and used it with 1st edition. It was published in 1986.
Under the spell Explosive Fireball, it says:
"Can be thrown at a wall, spot of floor, etc. (at +4 to hit) to catch foes in the blast."
- GURPS Fantasy, p. 20
That is carried forward to GURPS Magic, 1st edition, and eventually got clarified to mean a single hex. So, why is it +4 to hit a one-yard hex? Expansion from this "spot of floor, etc." from 1986.
Generally I ignore this for direct-fire, non-lobbed attacks. I just don't see why a 3' circle is a +4 to hit, especially when monsters that are 2 wide by 1 tall aren't even +4 to it. Either way, this seems to be the earliest reference.