The Foundation Setting Pill was designed to increase success in completing the 13 Rites. It was advertised as being capable of increasing one's Rite completion stage by one. Of course, this didn't include the final four Named Rites.
Though, Ryu believed that the reason it couldn't' do so wasn't because it was incapable, but rather because those that concocted the pill couldn't bring it to a high enough level.
The final four Rites were the most powerful and important of them all. If one wanted to become a true expert in the future, these Rites were the ones you would have no choice but to take a firm hold of. However, if you wanted external help to reach such a stage, the Foundation Setting Pill would have to at least be on the Runed level. Anything less and you could forget about crossing the Named Rites with external help. And, it was probably impossible to cross the final 13th Rite without the support of a full fledged and perfect Runed Pill.