The answer is 0.

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Let the initial term and the common difference of the AP be "a" and "d" respectively. Eighth term of an AP is a + 7d and its sixth term is a + 5d

a + 7d = 2*(a+5d) = 2a + 10d

a = - 3d -----(1)

Fourth term = a + 3d = 0 from (1)