I haven't messed with these ECU's, but on the Chrysler ECU's that I have messed with, generally knock tolerance went up with RPM. In the Chrysler turbo ECU's from the late 80's/early 90's, at idle to about 2K RPM anything over .5V will trigger knock and it's pretty much a linear scale up to about 6K RPM where about 3.9V is allowed before timing gets pulled.

Based on this, if you're seeing over 4V on the knock sensor, I would say you are probably about to put a hole in your pistons, or at least break some rings due to all of the detonation it would take to cause that much knock.

Again, this is just my experience with the ECU for the 4 cylinder Chrysler turbos so the SRT6 may be different and might not be as tolerant to knock (or more tolerant).