Question: Other Olympus digital voice recorders have a wider frequency response than the DS-4000. Why is the frequency response in the DS-4000 lower?

The DS-4000 is designed as an Olympus Professional Dictation Device, whose primary function is to record dictation files for transcription. Dictation recordings do not require a wide frequency response because the audio signals they are intended to reconstruct (tones produced by the human voice) fall within a relatively narrow band of frequencies.

Accordingly, the DS-4000 makes files that contain only as much audio detail as is necessary to re-create the original signal. By restricting the frequency response, the DS-4000 is able to create smaller files, which has the benefit of maximizing memory capacity (recording time).

The DS-4000 is not recommended for recording music or interviews intended for broadcast where a broad frequency response would be desirable.
Verification Source: Olympus Website