@Joseph,
(A.) Judges 20:6 certainly is singular, in form, (without Tav/ת), and affirmed by the context.
(B.) However, your presupposition that Job 4:15 is singular doesn't seem to have any basis;
(C.) You acknowledge that "flesh hair" is in the Construct State, as, משערות ראשי, (Psa 40:12), which is a collective noun, implying plurality;
(D.) Further, "hair" in Job ends with a "Tav/ת", implying plurality, (absence of Vav/ו doesn't change this)--and is the same in 2 Sam 14:11, (מִשַּׂעֲרַ֥ת), which is also plural, though the English word "One" is wrongfully injected. —
e.s. kohen 19 mins ago