vinegar to go from 8 to 6
baking soda to go from 4 to 6
Some apple cider vinegar could perhaps help to decrease soln/media pH. But baking soda/sodium bicarbonate isn't ideal for plants as it has too much sodium. It seems like it would take quite a bit of baking soda to bring the pH of a medium up from 4... and it might not last very long since NaHCO[SUB]3[/SUB] is water soluble. There is ample sodium in other things I\others use (sea kelp, blackstrap, Azomite, Excelerite...even coco coir in some instances).
Wetdog knows what he's talking about; dolomitic garden lime is one of the best things you can add to a
potting mix\soil. It works over time,in concert
with the soil neutralizing acids (H+ ions) there-by keeping pH in check and saturating the humus\clay or coco\peat particles with Ca and Mg cations that the plant needs.
Just like he mentioned, soil pH potentially changes itself even if you try to keep everything going in 'perfect'. Plants will change the pH of the rhizosphere over time by taking up nutrients, depending on which kind of nitrogen is available, and soil chemistry will further affect the pH of the over-all soil solution: often causing it to rebound.
Baby is here gotta go.