c4 on the harv is a good idea, but not to take the refinery out. a timed c4(the most powerful) does about a half-bar's woth of damage when placed on the outside of the building-and none when not in direct contact. If you placed it on the harv, it would do nothing, as the harv does not make direct contact(it appears that it does, but even those arms don't really touch).If you wanna disrupt their income, it's worth the tib damage to plant a couple timeds on the harv and take that out(and it's funny to see c4 on wheels...bomb go up...bomb go down...bomb go up...bomb go boom.
), that limits their income to the 2 cred mini-injects you get.