"The eleven Brazilian boys are the fourth."
Translation:A tizenegy brazil fiú a negyedik.
So ordinal numbers are exempt from the rule that adjectives following the noun must agree in plurality with the noun?
This sentence has a singular noun. Nouns modified by numbers or other quantifiers like sok will be in the singular, and the verb or predicate will agree with that.
Both "fiú" and "negyedik" are singular, no rule has been violated.
In the plural:
"A fiúk a negyedikek" - Both "fiúk" and "negyedikek" are in the plural. Again, no rule has been violated.
Now, on the "eleven boys". That's what jsiehler says. A number will not make the noun grammatically plural. It stays singular.
So therefore "negyedik" has no choice but to match the singular noun and stay singular.