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Velocity of sound at temperature T°C is given by

V=20.05 \sqrt{273.15+T }

temperature at NTP = 0°C
V_0=20.05 \sqrt{273.15+0}=331.3\ m/s

Let at temperature T°C, the velocity become 4 times
V = 331.3*4 = 1325.2 m/s

1325.2 = 20.05 \sqrt{273.15+T}\\ \\  \frac{1325.2}{20.05}= \sqrt{273.15+T}\\ \\66.1=\sqrt{273.15+T}\\ \\ 273.15+T=66.1^2=4369.21\\ \\T = 4369.21-273.15=4096.06\ ^0C

So at 4096.06°C, velocity of sound will be 4 times that of velocity of air at NTP.