A cover-4 scheme that doesn't work properly can absolutely be a talent issue - which, in our case, can't be divorced from the sanctions. Corners in a cover-4 could play off receivers, but need the quickness and awareness to react to quick hitter passes - corners also play off because they aren't athletic or fast enough to recover if a receiver beats them off the line. The key to a successful cover-4 defense is tremendous athleticism in the secondary; unlike a cover-3 like we had under Bradley, a cover-4 puts much more pressure on the cornerbacks because they don't have strong safety help underneath. Look at Pat Narduzzi's Michigan State defenses - they almost exclusively run a cover-4, and it's successful because they've been able to recruit and develop talented corners and safeties that turn into first round draft picks. Narduzzi took a solid 3 or 4 years for his MSU defense to get up to speed, and it was to recruit the type of players that you need in that defense (and, for that reason, Narduzzi's defense at Pitt this year probably won't be fantastic because they don't have the athletes yet).
I don't think anyone could argue that through inexperience, lack of depth, and lack of talent that Butler didn't have the horses needed to succeed with that kind of scheme, and I don't think one could argue that the latter two have the sanctions as a cause. At the same time, I don't blame Butler in the least for experimenting with a Cover-4, as it's really one of (if not) the most adaptable defensive schemes for the way college football offenses are structured today. It just didn't work out both because of a lack of personnel and depth as well as perhaps not enough time to fully install it which resulted in the confused looks that we would see last season.