If we are relying on 4th down conversions against the weakest opponent on our schedule, then what does that say about how things will go against tougher competition? Not only will we need 4th down conversions, but probably longer distance to gain for the first down, which further decreases the odds we will convert.

You are running into the classic correlation causation error. Just because BYU used 4th down opportunities to keep a drive going doesn't mean they needed to or that they were forced to at the hands of the opponants defense. Teams make what in tennis is called unforced errors all the time. Penalties, poor reads, poor execution on a single play. The 4th down is a luxury for a stronger FBS team vs an FCS team due to the knowledge that the odds of stringing together two penalty ridden plays or two plays in a row with poor reads/execution are much lower. Many Big Ten, SEC and ACC teams this week who played against FCS opponants not only used 4th downs, but some of them failed to convert them against teams they were shellacking.