The sample POST form on the Sabre Dev Studio page has a query setup for a round trip flight from DFW to LAX. You can submit the form and get results. However, if I change the airport from DFW to LAS then I don't get any results. I'm wondering if it's due to the test api having a limited amount of flight data loaded.
I was getting results for flights from LAS/OAK and BOS/LAS just a few days ago from my application I'm writing but now it's a 404 every time so I figured I'd try the sample page again.
Update: It looks like the 404's happen with one-way flights. My tests with round trips seem to be fine.
Looking at the Sabre FAQ and this other SO answer from Bruno, I think this is the expected behavior on the TEST platform.