But the key is how different the 5-segment will behave at the same data points (for lack of a better term) as this 4-segment thing.
Exactly: with a 4-segment SRB rather than a 5-segment SRB you'll learn essentially nothing about how an Ares-1 would fly.
We know we can launch spam-cans into space, because we've been doing it for decades. The question is whether the Orion spam-can can be launched safely on the Ares-1 firework... and testing a fake spam-can on top of a different firework won't tell you much.