Designing FIR bandpass filter for 20 MHz with same response as for 4 MHz sampling without increasing order of the filter

I am trying to implement a narrow bandpass filter using MATLAB simulink block . I have a signal with narrow bandwidth of about 25KHZ within a band of 20 MHZ . I only want to bandpass the narrow band signal while rejecting remaining band . but when i design the bandpass filter with 4 MHz sampling frequency its out put is very sharp with maximum stop band attenuation while for 20Mhz its response is degraded and sharpness of the filter is effected . I cannot increase the order so how i can i achieve same response for 20Mhz as for 4 MHz without increasing order filter response is attached for both sampling frequency

Numerator3=firpm(280, [0 800e3 810e3 860e3 870e3 (20e6/2)]/(20e6/2),[0 0 1 1 0 0 ],[20 1 20]);

>> fvtool(Numerator3)

>> Numerator3=firpm(280, [0 800e3 810e3 860e3 870e3 (4e6/2)]/(4e6/2),[0 0 1 1 0 0 ],[20 1 20]);

>> fvtool(Numerator3)