We use the G3//B3LYP method to calculate structures, harmonic vibrational frequencies, and energies at selected local minima and saddle points on the C4H7 potential energy surface. These include the straight-chain C4H7 radical isomers (of which there are twelve conformers of five structural isomers: 1-buten-1-yl, 2-buten-2-yl, 1-buten-2-yl, 1-methylally], and 3-buten-1-yl), sets of dissociation products, and dissociation, isomerization, and conformational transition states. Analogy with smaller hydrocarbon systems reveals why, for some reaction channels, more than one transition state may be accessed from the conformers of a structural isomer en route to the corresponding products. We compare results of the G3 and G3//B3LYP methods for selected C4H7 species.