Reducing sampling rate by a non-integer factor
I can recommend the resample command in MATLAB. It's a combination of up- and down-sampling. To get from 12kHz to a sampling rate of 9kHz, you upsample by 3 and downsample by 4. resample just needs your signal and the two factors for up- and down-sampling.  Remember, when downsampling, to first low-pass filter out everything between 4.5 kHz and 6 kHz. Very little energy should exist above the resulting Nyquist frequency.