Best way to study 4U is heaps of questions and practice. Towards the end of the course you will find most questions are very similar i.e. standard textbook questions but the hard questions can be very difficult (especially last questions of every HSC paper). Textbooks are not enough though, all trial papers you can get your hands on should be done as well (at least attempt hard questions if you want to save time and skip easy ones). Best ones to do are probably:
SBHS
JRAHS
CSSA (must do as most school like to use old CSSA questions from 1984 - 1999 in their trials/assessment tasks)
SYDNEY GRAMMAR SCHOOL
INDEPENDENT
MOST HSC
More important, just try to get deep with the concepts because a lot of difficult questions require you to use concepts from vastly broad topics and this can be hard if you are not familiar with simple concepts from the 2U/3U course. Doing these papers will simply get you exposed to these difficult questions and may help you develop your thinking skills for 4U. This is because sometimes questions require you to do stuff you would NEVER think of doing therefore doing a lot of practice would help.
MUST get books:
Cambridge 4U Maths (Arnold & Arnold)
Terry Lee Advanced Mathematics 7th Edition (blue book) (probably the best one
)
Optional Books to buy include:
Revised 4 Unit Course for Mathematics Extension 2 (Jim Coroneos - blue/white book) - This book has a good range of difficult of questions but the font used and presentation overall in my opinion isn't great. Way too compact. I didn't get it by the way.
S.K. Patel Maths Extension 2 2nd Edition (blue book)
HSC Maths Extension 2 Practice Questions by Topic (blue book - Jones and Fisher)
Good luck
