Damian,
This is a known problem. Around S194 or so we switched to shipping a 5.x based NSDB. Some 4.x signatures are no longer present in the 5.x product because they were handled differently, so it is possible that you can look up a 4.x signature in IEV and it not be present. We are in the process of completely redesigning the NSDB to allow you to customize your display for 4.1, 5.0, 5.1 etc.... I'm not sure how or if we are going to handle the 4.1 signatures that did not get carried forward, as they no longer exist in the database. I will take the issue to the design team.