Yeah, you can use less than 4 points, but it will not usually give very good results.
I was used to seeing the waveform change, as I adjusted envelopes, before I started with Reaper. The top part Audacity shows does not really mean anything, though. It is not like you can change the top and not the bottom. It might make more sense in terms of automating plugins. The envelopes are still basically the same, but all the way up is active and all the away down is bypass, or the other way around? In that case it is more like a square wave.
I make heavy use of volume envelopes. They are great for smoothing out when you initially set your gain not right and have to make quick adjustment when the set starts. If you get too much of a ramp, just add another point to round it out more.