Level 4 is a network engineer, which is not difficult to learn, and has little to do with level 2 C programming, so it doesn't matter even if you haven't passed level 2 or level 3. You can buy Xie Xiren's computer network foundation, which is the most authoritative computer network blueprint. First lay a good network foundation, and then buy NCRE computer network engineer counseling books, which are sold in major bookstores. You will read Xie Xiren's books, and then mainly read the questions in the counseling books. As long as you know the title of that exercise, you can recite it. In addition, I recommend Haibei's computer level 4 counseling book to you. Haibei's for the second grade exam, which has many original questions. It is definitely my sincere opinion, I hope it will be useful to you!