Concept:
Beat frequency:
\[
\text{beats} = |n_1 - n_3|
\]
Step 1: Use given relations.
\[
n_1 = 1.03n_2,\quad n_3 = 0.99n_2
\]
Step 2: Apply beat condition.
\[
|n_1 - n_3| = 4
\]
\[
|1.03n_2 - 0.99n_2| = 4
\]
Step 3: Simplify.
\[
0.04n_2 = 4 \Rightarrow n_2 = 100\ \text{Hz}
\]
Step 4: Find $n_3$.
\[
n_3 = 0.99 \times 100 = 99\ \text{Hz}
\]
Step 5: Final answer.
\[
(n_2, n_3) = (100,\ 99)
\]