p4 or p5

It is impossible to say which one is easier because every person is different. How easy or hard did you find F9? I ask because P4 repeats much of F9 but to a higher level. If you found F9 very tricky then maybe it would be best to avoid P4. I suggest you have a quick look through one or two past exams for both papers to see the style of the questions (the amount of calculation as against writing, for example) and see which suits you better.