Isabel last year was a Cat 5. But, it was only a Cat 2 when it made landfall.
Isabel -- like Floyd in 1999 -- were monster storms at one point. But they weakened before landfall because they were in the process of recurving. That is common in many recurving systems. The problem here is that IF Frances does indeed just truck W or WNW under a solid ridge, the influences that come into play to weaken recurving storms (trough interaction, inhibited inflow, cooler SSTs further N, etc.) won't influence her. In other words, she could MAINTAIN Cat 4 status all the way to landfall. Not saying it WILL happen, but if she gets to that level and the 120-hour forecast track of the NHC just gets extended out over the next three days in the same direction, we here in SE FL are going to be in seriously deep you know what.
Again, I am NOT saying that will happen. It is too early to say so. But by Sunday night and into Monday, the picture will be a lot clearer.