These would be better names from a purely phonological point of view: Unusper, Tresper, Quattober, Sexper. That way you don't have awkward clusters of both voiced and unvoiced consonants. The -ber affix having originally come from mensris (-mens-ris > *-membris > -ber), devoicing the initial 'b' to 'p' in those circumstances is reasonable enough. 'Quattober' is a simplification that makes it fit the pattern better.

Alternatively, you would rely on the ordinals, dropping the -us ending and replacing it with -ilis as in Sextilis and Quintilis.

  Unusber

It might not be correct, but I would prefer "Unober"

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