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At f/8 the mirror is probably spherical and no, it doesn't matter because of the long focal length. I have a table that give the min focal length for a 4" mirror to be usable as spherical as f/7.0 and a 5" as f/7.8 so your being in between those two aperture and longer in focal length than the 5" would easily be so. This is why these scopes and mirrors are so cheap - they don't need the specialised skills needed to make it parabolic!
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