The literal translation of the conversation is meant to make it look like Sam has a book so rare that there are only 4 in existence. By saying, "I'm always looking out for the other 3", it implies that he already owns one.
First of all, keep in mind that Sam is new to the area and looking to fit in. If he makes it look like he owns a comic as "serious" as Batman #14, it would make him look cool. And Edgar and Alan are in his peer age, so he's looking to gain some quick credibility.
Second of all, it's somewhat of a movie goof. Batman #14 isn't really that rare, nor is it even as valuable as other early batman books (Batman #16 is even more valuable, in fact), and no where near as valuable as Batman #1 (which is currently valued at $120K for a mint copy).