A comma splice occurs when two independent clauses are separated by only a comma. Looking at the sentences, the first one is fine because it is an independent clause followed by a dependent clause. The third sentence is really one independent clause with an appositive to describe the friend. The fourth sentence is just one independent clause. The second sentence though has two independent clauses. You can tell because in the first part of the sentence before the comma, there is a subject and verb; the second part also contains a subject and verb meaning they are not dependent on each other and therefore a comma splice.
