High pass filter seems not to work
If you want a filter that passes 18 kHz but blocks 4 kHz, you need to specify the minimum amount of attenuation that you require at 4 kHz. This will determine how complex (e.g., number of "poles") your filter needs to have.

For example, if you expect to have, say, 60 dB of attenuation over that ~2-octave span, you'll need something like a 5-pole filter, which will give you 5 &times; 6 dB/octave = 30 dB/octave.