Hi there!
[tex]\large\boxed{f(4) = 12}[/tex]
We know that:
f(1) = 6
F(n) = F(n - 1) + 2 (Meaning, the next "n" is 2 added to the previous)
We can count up to f(4) using this logic:
f(2) = F(2 - 1) + 2 = F(1) + 2 = 6 + 2 = 8
f(3) = F(2) + 2 = 8 + 2 = 10
f(4) = F(3) + 2 = 10 + 2 = 12