Let \( S_n \) be the sum of the first \( n \) terms of an arithmetic progression. If \( S_{3n}=3S_{2n} \) then the value of \( \dfrac{S_{4n}}{S_{2n}} \) is:
\( 8 \)
\( 4 \)
\( 6 \)
\( 2 \)