In Old Church Slavonic numerals didn't constitute a distinct part of speech, numbers 1-4 were numerical adjectives, numbers bigger than 5 were numerical nouns, that is why numbers 1-4 agreed in gender and number with the following noun, the noun being in singular after 1, in dual after 2, and in plural after 3 and 4:
- єдинъ чловѣкъ (m.), єдина жєна (f.), єдино сєло (n.) - Singular
- два чловѣка (m.), двѣ жєнѣ (f.), двѣ сєлѣ (n.) - Dual
- триѥ чловѣци (m.), три жєны (f.), три сєла (n.) - Plural
- чєтырє чловѣци (m.), чєтыри жєны (f.), чєтыри сєла (n.) - Plural
Numbers bigger than 5 were numerical nouns, so the nouns that followed them were attributes in the Genitive case, and, naturally, in the plural:
пѧть чловѣкъ (m.), пѧть жєнъ (f.), пѧть сєлъ (n.) - Gen. Plural
Later, when numerals formed as a part of speech and the noun declension paradigms got leveled by analogy, the gender distinction of numbers 3 and 4 got lost, and the noun forms after numbers 2, 3, and 4 became the same (namely, always ending in -a).
The traces of this old state of things which is reflected in the Old Church Slavonic examples I gave can be seen in all the modern Slavic languages, although the leveling went on in slightly different ways in each of them, for example in Ukrainian after 2, 3, 4 the Nominative Plural is used, only the stress in that case is the same as in the Genitive Plural.
