Aside from taste considerations, increasing salt in that range will serve to strengthen the gluten structure. Salt also retards the digestion of protein by bacteria in the starter to protect the gluten structure.
But you ask about the effect on yeast fermentation. In theory, salt dissolved in water will draw water out of the yeast cells, but in the presence of plenty of water, water will pass back into the cells. The permeability of cell walls in yeast is dependent on a number of other factors so it seems to me a very simple experiment would quickly answer your question - say prepare little batches of dough with 3%, 4%, and 6% concentration and the observe rate of fermentation as measured by volume increase.