IQ demodulation from specific rate
To get IQ data from a bandlimited signal centered at f0, with a bandpass well inside f0-f0/2 to f0+f0/2, and using a sample rate of fs=4*f0, you can simply keep 2 successive samples out of every 4, and ignore the rest.  Take every 4th sample, put them in the I channel, take the samples right after those and put them in the Q channel, and throw away the 3rd and 4th sample out of every 4 samples (I Q ignore ignore I Q ignore ignore I Q ignore ignore ... etc.).  The result is an IQ signal at a sample rate of f0.