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I see that in a 4 electrode ECG (LA, RA, LL, RL) we can measure upto 6 Leads (LeadI, LeadII, LeadIII, AvR, AvL, AvF).
What increase in value does adding an additional chest electrode like V2 provide?
What percentage of the heart activity is captured with a 4 electrode system vs a 5 electrode system?
Wilson leads (normally V1-V6) are so called unipolar leads: that means that 3 extremities' electrodes are put together over a high resistance to a so called "indifferent" electrode, against which you can record electrical activity to pretty much any point of the body. So, as you correctly stated, you need 3 extremities' leads RA, LA, LF (+ ground RF of course) for the imaginary 0 electrode, if you add a V2 you will get a lead with a main vector pointing from approximately middle of the chest towards the 4th ICR left parasternal (position of V2 electrode). This won't give you much information though since you can't really compare this lead to any of vertical leads (Einthhoven and Goldberger), that is why you do 6 Wilson leads to display the whole horizontal vector projection.
As for your question about the "percentage of heart activity", it is a slightly wrong understanding of the idea of ECG. Technically you just need 2 electrodes to record the whole electrical activity, the question is what those 2 electrodes are and what does their recording show compared to other leads. For a complete, unspecific description of heart activity the standard is a 12-channel ECG.
Decide if you wish to dig into the electrophysiological background of ECG, for a complete understanding of this examination technique you should do that, here's a good example of a book you can use (it's in German though), or take any other good physiology textbook Lehrbuch Physiologie; Hans-Christian Pape, Armin Kurtz, Stefan Silbernagl; 7. Auflage; Stuttgart; New York : Georg Thieme Verlag, [2014]
