Maybe
SeedRandom[314];
lists = RandomInteger[10, {5, 4}]
(* {{5, 7, 3, 9}, {6, 8, 5, 10}, {2, 0, 2, 5}, {6, 0, 10, 5}, {1, 8, 0,
8}} *)
Flatten[lists]
(* {5, 7, 3, 9, 6, 8, 5, 10, 2, 0, 2, 5, 6, 0, 10, 5, 1, 8, 0, 8} *)
Union[Flatten[lists]]
(* {0, 1, 2, 3, 5, 6, 7, 8, 9, 10} *)