I have lightroom 4.4 which I have updated and now uses Camera RAW 7.4. I am very happy with this version fo lightroom other than the fact that I do not seem to be able to download RAW files taken on my new a6000. I was under the impression that once I had bought a copy of Lightroom it would continue to be updated to remain compatible with new cameras - have I misunderstood. Do I now need to buy lightrrom 5 to be able to use my new camera?
You can either use the free DNG converter (version 8.4 or later), or you can purchase Lightroom 5.4 or later.
Unfortunately, you have misunderstood. Lightroom continues to be updated, and must be updated in order to support new cameras. Support for your camera was added in Lightroom 5.4. Lightroom 5 would be a paid upgrade for you. All of the subsequent Lightroom 5 updates would be available to you.