MUT0002 It's pretty much up to you; more prep is always better, but don't overdo it.
Yeah, you probably wont know when you are overdoing it till afterwards, but thats all part of the fun
Bear in mind your Unit 4 SAC is not inconsequential; it is a quarter of your final result. If you feel comfortable with the topic then by all means start exam prep.
If your exam prep involves practise exams even better; its covering topics from Unit 4 and simultaneously preparing you for exams.