In a open organ pipe the frequency of the wave is n = Vt/λ where Vt is the velocity of wave at temperature t and λ=2L is the wavelength of the vibrating wave. If temperature of air inside the organ pipe changes, the velocity of wave also changes, since V ∝ √T.
n = V172I where L = length of the pipe
Since beats decrease first and then increase to 4, the frequency of the pipe increases. This can happen only if the temperature increases. Let t be the final temperature, in Celsius,