Answer
$T=2.25ms$
Work Step by Step
The difference between two slightly different frequencies is known as the number of beats.
Mathematically, $f_1-f_2=No.of beats$
putting the values
$f_1-440=4$
But we know that by definition $f=\frac{1}{T}$
thus $\frac{1}{T}-440=4$
$\frac{1}{T}=4+440=444$
$T=\frac{1}{444}$
$T=0.00225s=2.25\times10^{-3}s=2.25ms$
$T=2.25ms$
