You can usually get at least 40-42" usable width of fabric, so one yard of fabric is more than one square yard. That's the problem with dividing by 1296. Two yards is only 72", which is 14" shy of 86", so that's why 4 yards won't be enough.
It sounds like you will need two lengths of 86" x WOF, but that will probably not be quite wide enough, depending on the WOF. How much are you willing to piece the back to get a full 86" in both directions? Could you use scraps from the front to create a stripe down the back?