problem with deleting certain frquencies in wav files

i am working on .wav files to filter them with cut off frequency 4 kHz, first i load the .wav file then i use the fast Fourier transform to get the signal in frequency domain, now i want to delete all the frequencies greater than 4 kHz and smaller than -4 kHz (band limited signal), my problem is when i delete the frequencies above 4khz and return the signal to time domain using ifft and save the file , the file works without problems , but when i delete both the frequencies greater than 4 kHz and smaller than -4 kHz the file does not give any sound !!!! ? *i delete the frequencies by setting their components to zero not by a filter.

[Walter Roberson]

You are removing frequencies smaller than negative 4 kHz ?? I'm not sure what that would mean?