I don't get this. For 0 - 4 Hz filtering, you need a high pass filter; however, it will be quite difficult to make a filter that cuts off at 4 Hz with a steep slope. What you would make here is an AC coupler with a large coupling capacitor; this will effectively reduce the throughput of low frequency signals, but you will get a reduction of signal strength up to a much higher frequecy as well.
On the other hand, I don't see how you will reduce car noise with such a filter. As a general rule of thumb, normal speech covers a frequency range of 300 Hz to 3 kHz. You should make a bandpass filter for that, which is more feasible. A low pass filter with a cut-off frequency of 3 kHz and a high pass filter with a cut-off at 300 Hz in series will do the job. You may want to make a filter with 12 dB attentuation to get the best effect with just a hand full of components. With a little Googling around you can find tonns of designs, both active and passive, on the internet.