Unit digit in 3⁴ = 1 ==> Unit digit in (3⁴)¹⁶ = 1

Unit digit in 3⁶⁵ = Unit digit in [ (3⁴)¹⁶ x 3 ] = (1 x 3) = 3

Unit digit in 6⁵⁹ = 6

Unit digit in 7⁴ ==> Unit digit in (7⁴)¹⁷ is 1.

Unit digit in 7⁷¹ = Unit digit in [(7⁴)¹⁷ x 7³] = (1 x 3) = 3

Required digit = Unit digit in (3 x 6 x 3) = 4.