Since a Kaon's wavefunction is composed of $K_S$ and $K_L$ which are CP-even and CP-odd respectively. The Kaon can decay into $\pi \pi \pi \pi$ through $K_S$ since the 4 pions have CP-parity $= (-1)^4 = 1$
Is this just like the explanation as to why it can decay into 2 pions?
Also, any recommendations on another book for particle physics other than Griffiths? It is really bad.