Maybe I'm using the wrong guides. When I go to support at the top of the page and then select one of the patton guides, the pages still reference 5.3. See here: http://www.3cx.com/voip-gateways/patton-sn4960.html. It states the current supported version is 'Smartware 5.3 Build Series 2009-03-18'.
On that specific model that is the supported firmware. But we have tested 5.4 on other models and it passed all tests so it should be fine. Try it and see how you go.