Looking at that page, I’m not buying the claims. I’m not an expert on Slovene, but my understanding is that it only has singular, dual and plural and not the 3–4 special case (which seems to have been confused with other Slavic languages like Czech and Slovak). I don’t think their dual rule is correct for Slovene either.

I’m surprised about the higher in the conversation comment about Greek as the dual exists in Modern Greek only as a grammatical feature of the written form of the word for “two” and is rare in classical Greek.