What's wrong with doing both?

I think that working on your 4 table game taking notes, AND trying to increase your max tables per session as a winning player to both be equally important to a point.

Granted, I only beat 5 tables of 5nl for about 20 bb/100 right now, and I'm getting close to moving to 10, but there are days where I want to get my hands in, but I want to go on autopilot and see if I have the type of game where I can beat 12 tables.

Most days, I want to take the best notes I know how, and pick bad players to try to pick on.

I don't think there's EVER a point where multi-tabling fails, but there certainly is a point where it doesn't make sense. And that's when you win more playing one table, than you do playing multiple tables. There's also a point where multi-tabling makes no sense too. Thats where you make more money playing X tables than you do playing X+1 tables.