Step 1: Node condition.
A radial node is where the wavefunction is zero, which from the given form needs $4 - \rho = 0$, so $\rho = 4$.
Step 2: Substitute $\rho$ for $n = 3$.
\[ 4 = \frac{2Z}{3a_0} r \]
Step 3: Solve for r.
\[ r = \frac{6a_0}{Z} \Rightarrow Y = 6 \]
Step 4: Answer.
\[ \boxed{6} \]