To calculate the forward rate between years 4 and 6, we will use the formula:

$(1 + f_{n,k})^{n-k} = \frac{(1 + y_n)^n}{(1 + y_k)^k}$

Where:
$f_{n,k}$ = forward rate between time $k$ and $n$
$y_n$ = $n$ year spot rate
$y_k$ = $k$ year spot rate

From the given information, the forward rate between years 4 and 6 is given by:

$(1 + f_{6,4})^2 = \frac{(1 + 0.04)^6}{(1 + 0.03)^4}$ $(1 + f_{6,4})^2 = \frac{1.265319}{1.125509} \approx 1.12422$ $`$1` + f_{6,4} = \sqrt{1.12422} \approx 1.06029 $$ f_{6,4} = 0.06029 \text{ or } 6.03% $$

Therefore, the forward rate is approximately 6.03%.