Sodium BIcarbonate is plain baking soda. The Na content of plain baking soda is not an issue.
My RO starts out with a KH of 0. I add 1/2 tsp of Sodium Bicarbonate to every 5 gallons of RO water to raise the KH to around 4-5 degrees. That's proper alkalinity for a planted aquarium and it all comes from sodium bicarbonate. I have no salinity issues and neither does anyone else that uses it. It's safe.
The ferts we add to our freshwater aquariums are salts. The Equilibrium that I use to reconstitute my RO water contains minerals in the form of salts. They aren't the same thing as adding table salt or aquarium salt though.
Now, if your KH is already at 4, it's pretty much perfect. Leave it alone and consider yourself lucky. Save the baking soda for the fridge and the occassional teeth whitening