[math-fun] Number theory pattern?

Consider the sequence s_n := (4^n-1)/3, n = 1,2,3,.... Back of the envelope shows that at least for very low n, s_n is squarefree and always has a prime factor that's not a factor of any previous s_n. Do these patterns continue forever, and if so, why? This is OEIS A002450 <https://oeis.org/A002450>, but these features are not mentioned there — so it seems likely they're both false. —Dan