Lemon Juice is what I have found is best for lowering pH. I use Apple Cider Vinegar sometimes, but it doesn't maintain the pH for long before it climbs back up.
But listen, and I promise this is good advice...if you are growing organically, you don't have to bring your pH back up to 5.7. Your soil (or soilless mix) is doing that for you. 4.6 is just fine. You do have some lime in your soil right? That will keep your pH stable as well. As missnu just mentioned, you don't need much CaMg unless you're using RO water. And if you are, I'd recommend getting a product like mineral matrix or something to use sparingly as well.
If you are growing organically, you also don't have to worry about lowering your pH. This is why we grow organically, we give the soil what it needs and it does the work. Now that being said, I use .5 tsp of organic molasses per gallon every watering, with or without nutrients. That lowers the pH of your tap water a little right there. I also add .5 tsp of organic lemon juice per gallon just because my tap water comes out at over 8.5 after it's ran through my small boy and most people's tap water pH is a little lower. So when I just water, or when I'm watering my pH is still around a 7.0 and when I have nutrients mixed in it's usually 4.0-5.0, just like yours. No worries man.
