Well, if I don't switch to 4e, then I don't care. I'm probably not in the market for game books, period, so it doesn't matter if they go 4e, stick with 3.5e, go 3.75, go non-d20, or something else entirely.
However, if I do switch to 4e, then the companies I will be looking for are Paizo, Goodman and Necromancer. I would expect to buy at least one, and probably more, product from each of these during the lifespan of 4e. I will also be very interested to see which way Green Ronin and Mongoose go.