The lens says its a 4x5, what makes y'all believe this is a 3x4?
I know you have already figured it out but...

I have a 3x4 RB Series B and it's lens says 4x5.
I have also seen plenty of 4x5 Graflex SLRs that have a 5x7 lens.
Maybe it was common practice to put a lens one size up on these cameras.