I agree with the litmus test premise with one caveat: this is the last game we will be without Rodney Harrison. He will be coming back lean, mean, healthy, PISSED, and with something to prove. So no matter what happens against Cinci I think our DB corps will be getting a big boost week 5, if only in terms of veteran leadership, and keeping a fresher rotation out there. Rodney went all the way through training camp and the preseason before being suspended, so he shouldn't take much time to get back into the flow. And it's not like he's returning from an injury - he actually said during preseason that he was feeling fully healthy and back to "himself" for the first time since 2004.
I also think we are going to start seeing Asante getting closer to the Asante we saw last year. Let's not forget that UNLIKE Rodney, Asante did not participate in camp at all, so weeks 1 and 2 were like the first two preseason games for him. Week 3 he got his first pick - I expect bigger and better things from him as he gets more games under his belt.
OK, so that was two caveats. But my real point is that I think there are at least two good reasons to think our pass D to be even better AFTER week 4.