Hey guys, I'll share what I know from chemistry.
Cornishman, the reason your base lets off a color in the water isn't actually because the base is soluble, it's because the color that comes through from the processing are pigments called tannins. Tannins actually exist in the acidic state in nature, so it's Tannic acid, and we know from our knowledge that acidic things are water soluble (think #4 HCl) so that part would actually dissolve giving you the color.
Also, I call the brown base #2, as you're right #3 is just #4 made conducive for smoking. Look up eutectic mixture on wikipedia and it explains why (it actually just lowers the melting point below the combustion point, so in the end it vaporizes faster so it doesn't burn.) People call it #3 because they say #2= morphine base ===> heroin base. So they say, number two starts off as morphine base, therefore it isn't heroin base. So you see where that argument comes from.
We know that bases can still pass mucus membranes, just not very well (think sublingual benzos). While you're right, soluble things pass through very easily.
I would have to agree with Tommy when he says that it was probably some highly cut product that 80mg had and was probably just not washed up that well, so that gave it the brown color with those tannins staining it. Caffeine is a pretty common cut these days in just about everything so that very well could have been in the #4. Just since it was used as a cut doesn't necessarily mean it was #3.
Hopefully some of that info will be useful and is easy to understand. But yeah, check out the eutectic mixture thing and that explains why true #3 is made with caffeine.