Hi Helmut,
When computing SNR over such a low LPF frequency (4.02Hz), keep in mind that you are in the 1/f noise region of the OPA637. Therefore, depending on the Noise Analysis start frequency (where you've used 1Hz), you'd get a different SNR result.
Here is the SNR reading if you start at 0.1mHz instead of 1Hz (SNR= -4.6dB):
If you were not dominated by 1/f noise (i.e. if your LPF cutoff was far above the 1/f noise corner of the OPA637), the Noise Analysis start frequency would not have affected things as they are now!
So, to get the actual SNR in your special low frequency application, you must consider the lowest frequency that matters in your application. If this is a sampled system, the lowest frequency is somehow related to the sample and hold time of your ADC (although I'm not well versed on that- you may be able to Google it). If it is an analog only system, may be a good measure of the lowest frequency is how long you are willing to wait (seconds) for a measurement.
Regards,
Hooman