The US military always used C4 for the same purpose...we had (and I still have) Brit PE 4. However, plastic explosives don't burn as hot as fire fudge...
In any fire igniter compound I want something that burns hot enough to idry out and ignite a log of hard wood that is soaked.
Oh lovely. We've moved on from what could become a 'low explosive' (chlorate/sucrose) to
high explosives now.
I'm not sure I'd ever have a need to light a wet chunk of wood by itself, but if I did (and didn't have any C4 layin' around
) I'd use a thermite fire starter. You can make these from three safe components (powdered iron, aluminum and mag), and they will light with a visco fuse.
1/2 of a 35mm film canister of the stuff produces a nice little ~4300f puddle of molten iron that should light even the most stubborn piece of soggy African hardwood.
Some safety notes on these - You want to light them and stand a good distance away until they burn down, in order to allow any water in what they're placed on to boil off. They will spatter some of the material if steam forms under them. Never place them on concrete or rocks, even if they look dry. Concrete and some kinds of rock can retain water for a long time and can explode violently if subjected to high, concentrated heat sources.
Dang.... After writing that, I'm thinking that your C4 might actually be safer!