MAGNITUDE

 33 dB 

 
The 40 phon curve (which crosses the 40 dB line at 1 kHz)  dips again at
4 kHz to about 33 dB. This means you'd have to lower a 4 kHz tone by 7 dB to make it sound equally loud as the 1 kHz tone, which indicates that our hearing is more sensitive at 4 kHz.