In 2005, there was a reform in the system of College English Test, which meant college students would get their test marks and there would be no limit to the times they may attend the tests before they graduate. They can attend several times before graduation until getting the scores they expect. From my point of view, it's not necessary at all.
To begin with, CET only proves one's qualification, and it's only a measurement of students'knowledge of basic knowledge. It can not necessarily tell everything of one's English ability. Passing the tests can not prove that one can use English skillfully. It's the use of English that can tell one's English ability. Moreover, students should spend more time in their major courses, rather than in getting higher scores in CET. Preparing for CET is a huge job that costs a lot of time. College students should focus on improving their major skills and comprehensive skills. English is just used as a tool in communication, but the modern society needs those people who grasp diverse skills. The companies and enterprises will take the candidate's ability on major courses into consideration rather than only the CET scores.
From the discussions above, we can conclude that the score of CET-4 and CET-6 can't prove one's all-round ability as much as previously thought. Therefore, college students needn't to attend CET-4 or CET-6 repeatedly, and what they should do is try their best to improve their major courses and comprehensive skills.