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Why ?
The lower the focal ratio, the brighter an extended object will be. As will the sky background. This is not true for stars (point objects), where its aperture that counts. So yes f/4 would probably be discernably better than f/5, all other things being equal.
But there are a lot of other factors - the condition of the mirror and things that scatter light or affect the background (open vs closed tube) would all come into play - even the effect of having a white tube vs a black one, on your night vision.
Sounds like the filter you were using was a good match for the light pollution there.
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