you have to look at 3rd and 4th downs together not separate. It is a fact that if you intend to go for it on 4th, you will make different play calls and pass pattern choices on 3rd than if you don't intend to go for it on 4th. The difference between getting a first down and getting to 4th and manageable is often huge because defenses will let you get a lot of yards short of the sticks. If you are calling 3rd down plays to set up a manageable 4th down, then your 3rd down percentage will suffer.
Do you understand that point? I'm not asking if you agree with it. I just want to know if you understand it.