"The eleven Brazilian boys are the fourth."

Translation: A tizenegy brazil fiú a negyedik.

[Arcaeca]

So ordinal numbers are exempt from the rule that adjectives following the noun must agree in plurality with the noun?

[jsiehler]

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.

[vvsey]

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.