I know you've probably heard this hundreds of times, but it really depends on the person.
Those who excel at maths won't find c3 and c4 too difficult tbh as long as they put the time in. There is nothing too abstract which is hard to get your head around IMO. However A level maths isn't considered easy and a lot of work does need to be put in.
The hardest stuff people generally find in c3 and c4 are the trig in c3 and the integration in c4. Some of it can get quite tricky so I recommend doing a lot of work on this.
Overall I think if you are willing to put a lot of time and dedication into c3 and c4, and if you really go for it, by the time you get to the exam you probably won't find it too difficult and you'll do well. But don't take it for granted if you're not a genius don't expect to do well half assing it.
If you aren't already aware, I highly recommend using Examsolutions.net video tutorials to help consolidate your knowledge, and I recommend looking at their worked video solutions to exam questions.
When it gets near to exams be sure to do a lot of past papers, do all the papers for your exam board and do all of the Solomon papers.
Sorry if I am not directly answering whether it is hard or not, but how hard you find it ultimately depends on the work you put in.