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Are they both correct?

Hello!

I have learned that we don't use plural for hyphenated noun which is used to make adjective.

But today I also found an sentence that says there are cases to make such type of adjective "without hyphens" .

But the sentence doesn't explain more, so there is one question left.

In the case of without hyphens, we should/can just delete hyphens and leave spaces,

a 4-year-old boy>>>a 4 year old boy

or we should change the noun into plural?

a 4-year-old boy>>>a 4 years old boy

I seached several past similar questions, but I could not get the point clearly.

So, I humbly post here.

Thank you in advance.

blissful baby
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The answer is simple--

This is right: a 4-year-old boy.
This is right: The boy is 4 years old.

This is wrong: a 4 year old boy.

Please supply us with the 'cases without hyphens' for our examination.
Comments  
As far as I know only a '4-year-old boy' is correct (but 'the boy is four years old'). Perhaps they mean adjectives like 'electromagnetic', which evolved from the compound adjective 'electro-magnetic'.
 Mister Micawber's reply was promoted to an answer.