Are all of the following sentences correct? If yes, what are

Sentences (1) and (2) are correct and almost identical in meaning. "Should you" is more or less another way of saying "if you should."

Sentence (3) is not quite grammatically correct for two reasons. For one thing, the second half of the sentence sounds like a "then" clause ("then it is likely...") but the first half of the sentence does not contain a corresponding "if" clause. Second, since the first part of the sentence contains the phrase "you would discover," the whole sentence sounds like it is a continuation of some previous thought in which an improbable or nonexistent condition was being proposed; by itself, however, the sentence does not supply us with that condition. That is, it is awkward to use "you would discover" unless you also mention the conditions that make the discovery possible: "If you spent more time in the library, you would discover in yourself a fondness for great reading..."

Sentence (4) has the same "if-then" problem as sentence (3).

Wow! your reasoning is SO VERY helpful! I benefit from it a lot!
Thank you Shapeless. <beer>