Let a be the first term and d be the common difference of the AP. Then,
4 x a4 = 18 x a18 (Given)
\(\Rightarrow\) 4(a + 3d) = 18(a + 17d) [an = a + (n - 1)d]
\(\Rightarrow\) 2(a + 3d) = 9(a + 17d)
\(\Rightarrow\) 2a + 6d = 9a + 153d
\(\Rightarrow\) 7a = -147d
\(\Rightarrow\) a = -21d
\(\Rightarrow\) a + (22 - 1)d = 0
\(\Rightarrow\) a22 = 0
Hence, the 22nd term of the AP is 0.