I can assure you 100% that someone will have taken S4 and found it hard, just as much as I can assure you that someone will have taken it and found it easy.
Whether it is hard or not is a question that depends on your ability, and no one else.
How did you find S1 to S3? How was the progession? Those are questions that you can answer, and will be far more helpful to you than someone elses opinion of the module.
I even found S1 difficult. I got like 65% in this mock S1 paper which was sent off to be marked by real markers outside school. HOWEVER, I managed 97% in the real exam. I still find it hard though and Im so happy I dont have to do S2 in A2 as Im doing M1 which should be easier since I also do Physics. I wouldnt like to imagine how hard S4 would be if I find S1 challenging.