Honestly, I did very very little in terms of study for Physics. Read the text a few times, read some other notes, read some internet stuff etc. When it was nearing exam time, just do some practice exams. The concepts are not very difficult to grasp in unit 3, the calculations can be a little tough though. It's the opposite in unit 4; the concepts are bizarre but their application simple and repetitive. Although there can still be some hard questions, obviously.
The detailed studies are quite straightforward also, made particularly easy in the exam by the multiple choice format.
Ah..
Oh, and if you weren't aware you can bring a two sided A4 cheat sheet into each exam. This can be very useful, if you work it to your advantage. It can serve as a reminder of the key concepts, a database of weird facts relevant to the exam taht could be of use. You can put whatever you want onto it.
Personally I'd say it is quite a straightforward subject which can be very interesting. I didn't put too much effort in and still had the necessary understanding for the exams. I just make careless errors all the time. You need to watch out for these; they are the downfall of many students.
Other than that, Physics is awesome. Do it.
And no, 3+4 is a direct follow on from unit 1+2 apart from a few new areas which crop up in year 12, and also a few branches are left behind, such as radioactivity. However the basics are still covered quickly in 3+4 so you don't have to have done 1+2.