Thief Dirty Fighter Skill - Does it reduce normal damage?

I was wondering about the Thief's Dirty Fighter skill. The description says to add an extra +1d8 to backstab damage, and that all other attacks deal +1d4 (does not say an extra +1d4). Does this mean that the thief is lowering his normal attack to +1d4 in exchange for an extra +1d8 on his backstab?

I believe it means add a D4 to normal damage.