4, 28, 160, 990, 6970,55832, 502560

4 x 4 + (4 x 3) = 28
28 x 5 + (5 x 4) =160
160 x 6 + (6 x 5) = 990
990 x 7 + (7 x 6) = 6972
6972 x 8 + (8 x 7) = 55832
55832 x 9 + (9 x 8) = 502560