Before responding to the question, I did some quick research online.
Turns out that most indicate that a 4 ohm load is what it's designed for. Fender used these heads with two 8-ohm speakers wired in parallel to present a 4 ohm load.
That said, I don't think that running it at 8 ohms would hurt anything, and I have done that in the past. I'd expect that it would cost a bit of volume.
Perhaps PBG and some others will weigh in on this.