6, 26, 134, 666, 3334, 16666,?

6 x 5 -4 = 26
26 x 5 + 4 =134
134 x 5 - 4 =666
666 x 5 + 4 = 3334
3334 x 5 -4 = 16666
16666 x 5 + 4 = ?
? = 83334