In response to several questions about the problem I'm having.
1) I've never installed v5 (trial or otherwise). I didn't even know about v5 until the green pop-up started.
2) It's the visual pop-up that's a problem, I'm not getting any voice announcing the update (if I had, I would have probably turned off avast! completely!).
3) I realize the risks of manual updates, but it's a risk I consciously take. I update definitions frequently and scan often, but these are done on my schedule. With settings to manual, I've never had any pop-up from avast! until now.
4) I will probably eventually update to v5, but at this point I can't afford the time to do so, or even the (probably tiny) risk that there may be problems when I do so. So for now v4.8 is what I'm staying with.
The only thing I'm trying to find out at this point is how to turn off the green box without turning off avast!. I appreciate the advice on how to better protect my machine, but that is getting beyond the scope of my current problem. Thanks!