Concept:
For motion starting from rest with uniform acceleration:
\[
s=\frac{1}{2}at^2
\]
Step 1: Since the plane is smooth, acceleration down the plane is constant.
Step 2: Distance travelled is proportional to square of time:
\[
s\propto t^2
\]
Step 3: Let full length of plane be \(L\), and time taken for full length be:
\[
T=4\text{ sec}
\]
Step 4: For \(1/4\) of the length:
\[
\frac{s}{L}=\frac{1}{4}
\]
\[
\frac{t^2}{T^2}=\frac{1}{4}
\]
Step 5:
\[
\frac{t}{T}=\frac{1}{2}
\]
\[
t=\frac{T}{2}
\]
\[
t=\frac{4}{2}=2\text{ sec}
\]
\[
\boxed{2\text{ sec}}
\]