The recording feature that's engaged when you press 4 is not part of the OBi. It's a Google Voice feature that the OBi has no control over. I just checked and you're correct, Google Voice appears to ignore this setting on its options page when the call is going through the Google Chat/XMPP interface that the OBi uses. I suspect you'll find the same behavior if you receive the call in Gmail with the OBi disconnected.