It isn't particularly difficult to do well in four. If you have the ability to do well in three then doing well in four will be a question of effort, not ability. There generally isn't any need to study four and, given that you plan to study sociology at university, you are unlikely to be negatively impacted (for the most part) by studying both sociology and psychology. I think if you intent to target some of the more academically prestigious universities it would seem sensible to take all four if you are confident your work ethic will persist through both years but again, I wouldn't say it is necessary.
Best of luck.