Often times, understanding the content in 4u isn't enough. The really good students have a creative way of thinking in maths, and that's generally what's tested in past papers, it's not like 2u (or even 3u at times) were you can cover the content and be set for any question. 4u requires mastery + the ability to think outside the box.
In relation to your question, complex numbers, and polynomials depending on your strengths, are generally the "easier" topics in that most students can understand pretty much all the content. It's generally the more abstract topics like graphs and mechanics that get people. So to prepare for the future topics, learn to think abstractly.
My advice is that you keep doing those past papers, but also mark them and read over the solutions in depth. Try to then replicate the same techniques in another question. Always go into the question with an open mind and do not narrow it down to "I have to use this formula to get the answer".
Also, go on the 4u marathons on bos and try participating in them. They generally have quite a few challenging questions and test your abilities to do maths. Even just absorbing the way the other BOSers answer the questions should really help.