Answer: The most effective way to combine sentences 3 and 4 is using a comma followed by the coordinating conjunction "but" to join them: "His real name was Samuel Wilson, but his neighbors called him Uncle Sam".
Explanation: As (3) and (4) stand alone as sentences, they can be joined by creating a compound sentence, that is to say a sentence made up of two independent clauses. In that way, "His real name was Samuel Wilson" would be the first clause and "His neighbors called him Uncle Sam" would be the second one. However, in order for a compound sentence to be correctly written, a comma and a coordinating conjunction are needed. In this case, the coordinating conjunction "but" could be added between both clauses since this word is used to connect ideas that contrast. Therefore, the most effective way to combine sentences 3 and 4 is writing "His real name was Samuel Wilson, but his neighbors called him Uncle Sam".