Increasing returns to scale
What I believe you mean is that a production function has Constant Returns to Scale.

i.e. $$f(\lambda K, \lambda L)= \lambda f(K, L) $$

Where $\lambda \ge 0$ 

Thus in your case where $K=3$ and $L=6$

$$f(3,6)=3 \times f(1,2)$$ 

and since ${f_{max}(1,2)=4}$  (i.e. $f(1,2) \le4$) the maximum product a firm can produce using $K=3$ and $L=6$ would be

$$3 \times f(1,2)= 3 \times 4 =12$$

Hope this helps!