So, aquarium salt is not the same as marine salt. Aquarium salt is (I think) just sodium chloride and maybe some magnesium chloride thrown in. To the best of my knowledge, it doesn't help with buffering the pH, alkalinity, or a number of other factors that marine organisms (which I will, for simplicity's sake, also include brine shrimp under) require to stay alive. A clownfish, for example, would not thrive in an aquarium that had the equivalent amount of aquarium salt as marine salt. Marine salt, however, is a number of different salts. There's calcium and magnesium ions, but there's also potassium and bicarbonate/carbonate ions as well. You can hatch out brine shrimp no problem in aquarium salt, since the key points towards hatching the shrimp are light, heat, and water with a certain amount of salt in the water (doesn't matter what kind). But when you're raising the brine shrimp, they're adapted to all of the different salts that you find in marine salt mixes as opposed to just the sodium/magnesium chloride probably found in aquarium salt. As an aside, if I remember correctly, four days for me is around the point when the BBS start dying of starvation if you don't feed them the right foods. I'd suggest a live phytoplankton mix like OceanMagik or similar if you're culturing them out.
@Blue_Ridge_Reef, when feeding, I've found that if you're adding live phytoplankton, it doesn't really matter how heavily you feed, since the phyto does the job of helping filter the water as well. However, when feeding dead foods (yeast, spirulina, etc.), you just need to feed very lightly, and there's not really a need to filter the water as you would a fish or shrimp tank.

Also, a lot of people tend to overstock the buckets when starting out with brine shrimp culturing; really a drop or two of concentrated BBS is probably more than enough to get one started, as opposed to dropping in an entire batch worth.