Just by a quick glance: Bodybuilder/Athlete is based the rocket launch rep style: which means, you have to be ultra-explosive, so the moment fatigue sets in, you stop the exercise (little fatigue). 4 Days a week uses 4 far higher impact sessions: main lift+assistance+optional isolation can be cause a lot more fatigue and muscle damage. Therefore, rest should be taken more.
What does confuse me is that they both are billed as 'the best'. Although, they do slightly differentiate between 'best for building muscle', and 'best for strength...kinda' respectively.
I usually don't pay all that much attention to those claims: they sell a method, and that is a good thing! But after all, it's just another method. Hell, most templates on BB also claim they are the best. Difference is in what the programs deliver. And you know what CT consequently delivers:)