Confused at this question, from what I gather strong induction is necessary here to prove this but the algebraic step after the Inductive Hypothesis is where I'm not too sure.
2 <= a1 = 2 <= 4
2 <= a2 = 2 <= 4
Assume that for any 2 < n <= k, we have that 2 <= an <= 4
Show an+1 is true.