Does concussion get a +4 to hit the ground?

[Anaraxes]

In GURPS, targeting a hex at +4, instead of a single, moving, human in that hex, is a standard rule. (B414, for those GURPS owners that want a reference.) It applies to grenades, Molotov cocktails, tossing something to land nearby an ally instead of touching them, not just explosive spells.

The mentions in the explosive Missile spells are thus merely redundant, not the text that establishes the rule just for those spells. The +4 applies to any similar effect, spell or mundane.

DFPRG is of course not exactly GURPS, but I doubt it differs in this regard. Check the more general parts of the rules, especially where it talks about explosions and area attacks.