The sampling frequency doesn't directly effect the audio frequencies it can encode. Telephones do PCM encoding (meaning it has data representing the graph of the sound wave) at 8 kh/z. Following the nyquist sampling theorem (cut your rate by 2), this can allow frequencies up to 4 kh/z (as you said). It's not a hard cutoff though, you can still get most of the sounds above that pitch, they'll just sound pretty weird (as if you were talking on the telephone!)