Suppose two tuning forks are named A and B with frequencies ${\mathrm{n}}_{\mathrm{A}}=256\mathrm{Hz}$ (known), nB = ? (unknown), and beat frequency x = 4 bps.

Frequency of unknown tuning fork may be

${\mathrm{n}}_{\mathrm{B}}=256+4=260\mathrm{Hz} \mathrm{or} =256-4=252\mathrm{Hz}$

It is given that on sounding waxed fork A (fork of frequency 256 Hz) and fork B, number of beats (beat frequency) increases. It means that with decrease in frequency of A, the difference in new frequency of A and the frequency of B has increased. This is possible only when the frequency of A while decreasing is moving away from the frequency of B.
This is possible only if n_B = 260 Hz. 
$\mathbf{Alternate}\mathbf{ }\mathbf{method}\mathbf{ }$: It is given  ${\mathrm{n}}_{\mathrm{A}}=256\mathrm{Hz},{\mathrm{n}}_{\mathrm{B}}=?$ and x = 4 bps
Also after loading A (i.e. n_A $\downarrow$), beat frequency (i.e. x) increases ($\uparrow$).
Apply these informations in two possibilities to known the frequency of unknown tuning fork.
         n_A $\downarrow$– n_B = x$\uparrow$        ... (i) 
         n_B  – n_A $\downarrow$= x$\uparrow$       ... (ii)
It is obvious that equation (i) is wrong (ii) is correct so 
n_B = n_A + x = 256 + 4 = 260 Hz.