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.