There are two methods to change the frequency range.

Method 1: Change the frequency by re-sampling the data.

Select [Signal Processing] > [Resampling] in this order from the menu.

Change the "Re-sample Frequency" from 51200 Hz (20 kHz range) to 10240 Hz (4 kHz range).

Method 2: Change the frequency range in the analysis setting.
(This function is available from ver.2.7. )

Select [Waveform Analysis] > [Standard Frequency Analyzer] in this order from the menu.