\(n = \frac{p}{{2l}}\sqrt {\frac{T}{m}} \)
\(\Rightarrow n \propto \sqrt T \)
==>\(\frac{{\Delta n}}{n} = \frac{{\Delta T}}{{2T}} = \frac{1}{2} \times (4\% ) = 2\% \)
==> Number of beats = \(\Delta n = \frac{2}{{100}} \times n = \frac{2}{{100}} \times 100 = 2\)