The exam was fairly easy last year and so were the ones from the previous years. You can easily score a high mark on it. You don't need to prepare for it early in the year at all so don't worry.
But anyways I can tell you that to do well at the exams you just need to look through outcome 3 (their main dot points on the study design) for both unit 3 and 4 and ensure you know it. Other than that research on two artist and two artworks from each of them. Your teacher will tell you what you need to know of from their artworks (historical/social context of artist, styles (like Impressionism or contemporary), the date, medium and meanings behind the artworks etc).
As for your exploration proposal I recommend checking out the criteria sheet for it on VCAA's website as that is exactly what your teacher is going to mark your EP and evaluation against. It is also quite vital to constantly refer to your initial ideas in the EP and answer whether you've fulfilled them or not and if you've enhanced them in your evaluation and the REASONS for that. There is one special category there on the criteria sheet about mediums so don't forget to discuss about that too.
The criteria sheet seems pretty vague and scattered so I suggest highlighting out the main bits and then tackle them in your EP and evaluation.