No idea how it's officially called, but I'm not making this up :D

http://localization-guide.readthedocs.org/en/latest/l10n/plu...

I think i found the solution here : http://en.wikipedia.org/wiki/Dual_%28grammatical_number%29

It's the normal plural, but 3 and 4 take the nominative, while other numbers take the genitive plural.