Salem

I was tanning 4 hides and I put 2 buckets and tanning fluid into the tub. Because I only have a couple buckets and getting water takes porting to Boston, I put two more brains directly into the tub, making a 10% strong 20 liters of solution like the wiki says you can. However, the 4 hides didn't tan, only 2 of them did. So, I had to use another 2 buckets of tanning fluid to make it work. That's 6 brains needed for 4 hides. Obviously a bug.

Did you add two more buckets of water as well, or just two brains? Because you do need 4 buckets worth of water as well as the 4 brains.

To tan a single skin you need 10 litres of at least 5% concentrated liquid.
1 brain with 10 litres of water gives 5% concentrated liquid.
You had 20 litres of 10% liquid in your tub. That's enough for 2 hides. So, 20 litres were expended to tan 2 hides. And that's it.
If you added another 20 litres of water then you would get 40 litres of 5% liquid which would be enough for 4 hides.

Long story short - you wasted 2 brains