"The eleven Brazilian boys are the fourth."
Translation:A tizenegy brazil fiú a negyedik.
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.