Filter is filtering out part of useful signal (part of lower tone).

[zmikl]

Signal center frequency is 4950 Hz minus 170 Hz to get lower tone frequency is 4780 Hz, which is outside of the filter band pass. Low cutoff frequency as filter is designed is 4800 Hz. I have changed bandpass filter to 4700-5300 and I get better decoding result. I will experiment with other filters to get better result.

Did you try Goertzel decoding? I hear Goertzel is suitable for FSK.

Thanks for sharing your work.
Best regards.