For Edexcel, I found C3 to be easier than C2 (slightly biased as I am now a Maths student at uni) and C4 was.. significantly more difficult than C3, but nowhere near impossible to do. It's just the integration and vectors that requires a lot of practice, then you should be golden.
Personally I found C4 pretty straightforward once I clicked with C3 (I think the real leap in difficulty is C2→C3 rather than C3→C4) but I know people who have really struggled with the calculus in C4.
Obviously with practice, you'll start to find them easier but is C4 a lot harder to get the hang of?
I thought C3 was impossible but I find it really nice now.
C3 is straight forward providing you get to grips with trigonometry.
C4 is straightforward except for Integration & Vectors which take up some time to master. Expect to be using up a lot more paper for worked solutions on integration and sketch diagrams for Vectors as it helps significantly.
Overall be patient, consistent at past papers and eventually the grades will come through.
Yes, vectors are a nightmare, Reza. And I imagine that MEI was much more difficult than Edexcel for... well, every module.
I looked at some Edexcel papers when I was doing my A levels and ngl they seemed quite easy in comparison
Some say the grade boundaries tend to be higher for Edexcel which offsets it, but I disagree: fewer schools in general do MEI, and those that do tend to be better, thus a higher concentration of good students do MEI which means the MEI boundaries are higher too as a result.