Tbh most of the c4 past papers are easier than the core 3 past papers- questions tend to be more standard. Most of the way through m2 atm and it's not too hard. The actual content isn't particularly difficult although like mechanics 1, the difficulty probably comes from applying knowledge in exam questions. This year focus on c3 and 4 and depending on how you did at as, you don't actually have to worry about m2 too much.