Well I guess that 0-4Hz is just a typo. At that frequency you have to "feel" rather than hear. And the best of the Bose sub-woofers would struggle to reproduce it!

And even if it is 0-4Khz, what is needed is a low pass filter.

And since the end use appears to be a audio system, you can save lots of headache by simply buying a crossover filter ( select a 12dB / Octave type )that matches your needs and directly feed the amplifier output to it. That way the speaker handles only the required range.

OK all these are wild guesses in the absence of the knowledge on what you are doing.

Good luck.

Raghu