No, your right. Everything will be backed up and "reduced" if you have a blockage at complex IV (you can't keep passing electrons). But, I thought you said the question asked which part of the ETC would be stopped in methhemoglobinemia (which is essentially which part will be blocked in the absence of oxygen). Well, they all would be "backed up". But, the block actually occurs at Complex IV, no?
I guess I assume its the same thing as asking, what happens with cyanide poisoning. Well, you can't keep passing electrons along at the first complex because of a "block" in the chain. But, Cyanide actually does its damange at cytochrome oxidase