I found it a bit harder compared to Unit 4. For Unit 4, all you have to do is practise, understand, do lots of past papers and start early. For Unit 5, my exam board was AQA and we had to remember different colours--and I actually forgot all of them now--and I just found the concept harder. But it's different to everyone, some people will find it hard, some people will find it ok.
Exam board..? It'll vary a lot between different boards. For me, chemistry 4 wasn't too hard at all. Unit 5 was harder (in the exam), but I revised less for unit 5..
If you like organic chemistry you'll find it all right. I hate organic so ended up doing **** in chem4 but amazing in chem5 which pulled up my grade.
I must admit though, that when I looked back at Chem2 after finishing Chem4 it did look ridiculously easy in comparison. Sort of like the jump between A-Level and GCSE