A flat spot between 40 and 4200 RPM seems strange to me. I understand you to mean that the flat feeling goes away at 4300 RPM, right?
Is the problem purely RPM-related or does it also depend on the load, that is, whether you are getting to that RPM range in a high or low gear or on the highway versus around town?
Was your bike damaged anywhere around the engine when you hit the deer? What sort of damage did it receive in general?
Edit: Looking back, I see that the flat spot developed sometime after the deer hit. Maybe the two are completely unrelated.