Translation:Even a four year old boy would have found it.
This should be "a four-year-old boy" following the grammar rule: "When the age is an adjective that comes before the noun and modifies the noun, or when the age is a noun, hyphenate."
Actually this was the advice from the website "quickanddirtytips.com" (http://www.quickanddirtytips.com/education/grammar/are-you-using-hyphens-correctly
A quick search on google will give you tons of sites about hyphenation and style You might also like to check out Wikipedia (http://en.wikipedia.org/wiki/Hyphen)
I think descubrir in this context would often imply something more like "would have found out" or "would have figured it out" rather than finding a particular object literally. Lo can imply a situation rather than a specific thing (such as a car, ball, an so on). Encontrar seems to be for things that are more concrete and specific to me.
"even a child of four years would have discovered it" should be correct, but it was not accepted and instead Duolingo suggested "even a child of four years OLD would have discovered it". The extraneous word "old" makes the sentence sound very clumsy. It should be either "even a four-year old boy (child)" or "even a child of four years" (without the old).
"A boy for four years old" is incorrect English. The proper preposition there would be "of", though that whole phrase is awkward. "A four-year-old boy" sounds far more natural, and, in that particular case, when the age is being used as an adjective right before the subject, you would never use the plural of "years." There's no real reason why, that I know of. That's just how it is.
"The man is twenty years old."
"He is a twenty-year-old man."