I hate to be negative, but I think that enrolling to study Legal Studies Units 3 and 4 could be slightly difficult if you have not studied Units 1 and 2. I don't know you personally and for all I know you could pick up the subject very easily. However, I am currently undertaking Unit 4 Legal Studies and there are a few students in my class who did not do the subject in Year 11 and are finding it extremely difficult.
The reason for this is that there is an extensive amount to learn. The amount of background information you must know in order to undertake Units 3 and 4 Legal is crucial to your understanding. A lot of the content studied in Year 11 Legal is part of the study design for Year 12 and if you don't know it already, you may have a very hard time (in comparison to those who learnt it all the year before; to them it is merely revision).
I only say all this out of care. I'm now in Year 12 and am finding myself to be grateful towards the fact I did the subject last year. There is just SO much to learn and anyone who says there isn't is delusional or an exceptionally talented prodigy.
Anyhow, whatever you decide to do, best of luck with it. I'm sure that if you choose to do the subject and work very hard then you'll do well, but please do consider my response.