Would it possible to pluralize the numbers in bold in the following noun phrases? Why or why not?
As quatro cidades - As quatros cidades
Os cinco amigos - Os cincos amigos
Would it possible to pluralize the numbers in bold in the following noun phrases? Why or why not?
As quatro cidades - As quatros cidades
Os cinco amigos - Os cincos amigos
No, you cannot do that. In Portuguese only the numbers 1 and 2 are modified for gender. All other numbers are invariable with respect to gender. Thus:
*um lápiz e uma caneta* -- a pencil and a pen
*dois lápices e duas canetas* -- two pencils and two pens
*três lápices e três canetas* -- three pencils and three pens
*quatro ... e quatro ...*
...
and so on.
As @azafran341 said, apart from gender variability for numbers 1 and 2 (and any numbers ending in them), all numbers are otherwise invariable. Numbers above 1, by definition, already represent a plural amount as is, so there’s no need to further modify them.
They’re only modifiable in the few cases where they’re not being used to measure quantities directly, but as actual nouns. For example:
(@azafran341 just a correction: lápis* instead of “lápiz” or “lápices” → the word is spelled with S and is invariable )