I've done psych units 1-4 and I can definitely say you'll be able to do 3&4 without the knowledge of 1&2.
At the beginning you'll be reviewing some content from 1&2 in relation to the nervous system and research methods, but that's about it - everything onwards is new and almost completely unrelated to the first two units.
Like miniturtle said, in 3&4 you look at the nervous system, stress as a psychobiological process, learning, memory, sleep and sleep disturbances, mental health and wellbeing, phobias, etc. The content is waaayyyy more interesting compared to 1&2 so you're not missing out on much :-p
That being said, research methods is definitely a fundamental part of all units of psych and I know a few people who went into the subject without 1&2 and struggled a little with that topic, so that's just something to keep in mind (but, again, everyone is different)
I have friends that did bio units 1-4 who swear that 1&2 is necessary, while others believe 3&4 can be picked up easily; this, again, just depends on the person I guess.
I hope you figure things out!