Hi there, c.c.1. Diggin' the username!
I don't think you'll be at much of a disadvantage from not doing Psych 1/2; there is actually very limited overlap with Psych 3/4. In saying that, I did both 1/2 and 3/4, so my memory may be skewed a little. However, if I were in your situation, I would be focusing on dot points from the
present study design that are relevant to both Unit 3 and Unit 4. Some of these things are covered in 1/2; moreover, they are often brushed over a little during the year. As such, I think they're a good place to start. The dot points to which I am referring include experimental research, sampling procedures, data collection, statistics, ethical principles and so on.
Once you feel comfortable with these concepts (I would focus, in particular, on ethical principles), I don't think there is too much that you need to do. If you feel like it, you could read over parts of your textbook lightly; however, I wouldn't be too concerned.
The greatest resource you will ever find is, of course, the aforementioned study design. This is what I based all of my revision on; each dot point was a topic heading in my notes. Otherwise, you could try the
vTextbook. I've never really been exposed to the vTextbook, but from what I can glean, it's a fantastic resource. And
here you will find an online wiki that covers the entire 3/4 course.
Well done on choosing a brilliant subject! Perhaps others who have completed Psych more recently could add to this.

All the best,
Nick.