Running a sox spectrogram on a bandpass filtered wav file

Let's apply a 400-2000Hz bandpass filter on a respiratory .wav file:

sox audio1.wav audio1.bandpass.wav sinc -t 10 400-2000


Now I'd like to generate a spectrogram that takes advantage of the reduced bandwidth to still generate a full size image. But how to to do that? Notice what the result is from running spectrogram on the bandpass file:

sox audio1.bandpass.wav -n spectrogram -r -o audio1.bandpass.png -m


That's clearly a big waste of image real estate. What step(s) am I missing here to use the full image size to focus on that already-limited frequency bands?

Update From accepted answer: works great

Add a step after the initial bandpass:

  sox audio1.bandpass.wav -r 4000  audio1.bandpass1.wav


Then

sox audio1.bandpass1.wav -n spectrogram -o audio1.bandpass.png -m


However, as you can see, your signal is solidly oversampled: You can downsample it to a much lower sampling rate after filtering! (and downsampling by a factor $$N$$ here really is just throwing away $$N-1$$ samples, keeping one, throwing away $$N-1$$ samples..., since your filter is an excellent anti-aliasing filter.)